Hi there Jophis, and welcome to the forums!
You've received some great advice, and you will find a lot more to be had at this site. This is a fantastic resource with a very knowledgeable community. Many people own more than one BP, and there are several professional breeders who interact with other professionals and novices alike on the forms.
If you've just arrived here you may not be aware, but there is an excellent
Ball Python Care Sheet available. I found it immensely helpful.
I agree that keeping two BPs in one 10g may not be the best thing. For example, you mentioned that they were curled up together under one of the impromptu hide spots. This behavior is not really common. As humans our instinct is to filter what we see through our own experiences to understand them. So, we see two snakes curled together and we naturally assume they are getting along and cuddling. In reality, the snakes are silently vying for what they consider to be the "best" hide spot. The fact that the new snake invaded the hide where the other had already established itself is probably very stressful for him/her. It would be like having a stranger suddenly move into your bedroom and demand to share your bed!
Again, it is for reasons like this (which is just one prime example) that it is not recommended to house multiple Ball Pythons together. More reasons include the chance of spreading illness, or, especially in your case where you don't know the gender of this 'surprise snake', eventually unwanted mating when they get older.
Temperature
You will no doubt find most of your questions answered in the Ball
Python Care Sheet, but I do have a few points that might help.
As it has been stated, the hot side should be around 90-95F, while the cool side should stay around 80F, give or take a couple degrees. These numbers indicate
ground temperature – not ambient (overall) air temperature. The ambient air temperature should be around 80F.
When using a heating pad it is very important to have it plugged in to either a rheostat or a thermostat. Both of these devices are designed to regulate the power available to the heating pad so it will not get too hot. An unregulated heating pad can top-out at 115F – far too hot for your snake!
Rheostat: A rheostat is like a ‘dimmer’ that you can plug the UTH (under tank heater) into and then manually dial back the power current available to the unit. By using a digital thermometer you can check the ground temperatures of the ‘hot side’ until you reach your desired level. Keep in mind that it can take 2 hours for a UTH to reach its maximum heat output, so be patient. These units are not very expensive.
Thermostat: A thermostat, or more specifically, a
proportional thermostat, is a device that you also plug your UTH into to control the output temperature. However, these units can be programmed via a simple digital display to self-regulate the temperature at the desired level. They come with a remote temperature probe that you set on ground level on the hot side and the unit takes care of the rest. Good brands are Helix and HerpStat. I use
this HerpStat unit from
SpyderRobotics. Different models can moderate multiple UTH in different enclosures, too. These units are more costly.
Whatever UTH regulating device you choose, I also recommend using a surge protected and grounded power bar. I never plug my electronics directly into wall sockets. In the case of a UTH, one power-surge can drastically raise temperatures while burring out the unit and potentially killing your snake. I like Belkin power-bars and highly recommend them.
A final note about UTH units that stick to the bottom of tanks; do not reuse them. The manufacturer warns against this on the packaging for a reason, and not just to sell more units.

It is because the bending and twisting while removing the unit can damage circuits and wires which can contribute to malfunctions, ‘super hot’ spots and ‘dead spots’ on the pad.
Humidity
As hammerhead mentioned, 50-60% is considered proper, with 60-70% during sheds. Keeping the water bowl over the UTH can help raise the humidity, but if you are not able to maintain 50-60% try misting the walls of the enclosure twice a day - once in the morning and again before you go to bed.
Overhead Lights:
Being nocturnal Ball Pythons don’t need special lights that mimic the sun. However, if your ambient air temperature isn’t what it should be, than an overhead heat light can be helpful; 40wats for a 10g tank, 60w for a 20g.
Personally I found that the basking lamps really killed my humidity, so I opted for a Ceramic Heat Emitter (same wats per gal apply). They don’t emit light, just heat, and they don’t affect the humidity.
Terrarium size:
A 10g terrarium is fine for one baby BP. In a year or so (depending how fast it grows) you may upgrade to a 20g or 30g. I’ve started mine out in a 20g and will upgrade to a 30g for his adult size. If you divide your 20g in two with a terrarium divider, your snakes will be happier.
Hissing & Biting:
As hammerhead pointed out, hissing is a sign of ‘back off.’ It is important to leave your BP totally alone for the first week. It’s good to know that they are both feeding, but now is a time to let them chill out and get used to their new environment.
When a snake is going to bite it will usually ‘rear up’ first, so watch for that action. If your BP ever does bite you, try to not panic. Try to fight the urge to pull your hand away, because this can damage their teeth and harm the snake. Their bites, from what I’ve read (to date I’ve yet to be bit), are not painful so much as ‘surprising.’ They have no venom and no fangs – so just bare that in mind.
There are two types of bites; tags and latches. You can find lots of information about those types by searching the forums, but in a nut-shell:
Tags: This is similar to a dogs ‘nip.’ The animal puts no effort into the strike to cause harm, and usually only ‘touches’ you with the teeth before pulling away. I’d say it is equivalent to a human’s hand-slap. It is the last line of, “Get away from me NOW”
Latches: A latch is uncommon for Ball Pythons and usually only occurs due to being suddenly startled (defense reaction) or as a mistaken feeding attempt usually caused by an owner forgetting to wash their hands after handling feeders and then putting their hand in the cage. The snake smells food, sees a warm moving thing, and strikes.
If a latch happens it can be tricky to get the snake off. Usually they don’t last too long, so waiting it out is not uncommon to start with. However, if the snake shows no sign of relenting then you can try dripping some cold water on its head with your fingers. They hate that. More drastic measures include opening a bottle of rubbing alcohol and holding the top close to their head. They find the smell unbearable. Usually those two things will get them to release.
Again, search the forums for more information. More information is always a good thing!
On a final note, it is true that most babies can be ‘nippy.’ They are not doing it to be vicious, just because they are scared. Human babies cry, snake babies bite. It’s just their nature.
Handling:
After the first week of leaving them alone (starting when you set up their divided/individual enclosures) you can start handling for 5-10 min every so often. Maybe not even every day, but every other day for a week. After that you can increase the time slowly, up to maybe a half hour. As the snake gets older and grows to know you and your scent it will be relaxed and, for some, even happy to be handled.
I also pick my BP up by approaching him from behind so he can't see my hand directly. This is considered less threatening. If I am lifting up his hide to pick him up, I first lightly tap on the top so he knows I'm there. Before I lift him up I also gently pet his back with the back of my fingers. Slow gentle movements are a must when handling BPs, especially as babies. Don't touch their head or the first 4-5" of their neck, either. They
hate that.
Patients is a virtue with Ball Pythons!
Remember that security is a major issue for Ball Pythons. They need to feel safe and comfortable in their environment, and this means providing them with 2 hides: one on the cool side and one on the hot. Because they are nocturnal you will probably rarely/never see them during the day, but when comfortable they will start venturing out in the evenings.
>>> I am guessing that the one that came in the tank is calm because it is already familiar with the tank. I just can't believe that the pet store never took the snake out and forgot it was even in there.
I’d say you are right about the ‘surprise’ snake. I’m personally shocked at the negligence of the pet store for ‘overlooking’ an animal in a cage/setup they sold. Wow. I can only hope it was not left without fresh water and food for too long before you got him/her.
Due to this fact, you may seriously consider taking them to a vet in a couple of months for a checkup. Get a fecal test done on them to make sure they are not carrying parasites and they are overall healthy. A list of good herp vets can he found
by clicking here.
Please post pictures of your new BPs in the
BP Pictures Forum!
I hope this has been helpful.