I have dealt with vets who were very inexperienced with snakes/reptiles, I even had one try to tell me that beardies don't brumate and if mine is sleeping she must be very sick(not true). I also had a vet over medicate a snake because he didn't know what he was doing. Needless to say I don't go to those vets anymore. Most RI's are contagious through physical contact. For example you interchange hides, water bowls or even touch the snake's water and then touch anothers water. Some snakes have much stronger imune systems than others (like people) and don't get sick as easily. Also some forms of RI's remain dormant and pop up when a snake is stressed. PI's are not one just one germ or virus but a variety so it depends what you snake has to begin with. Chen=micals could also cause RI's. If your other snake is not showing signs then you may be real lucky or it is dormant/incubating. I know a breeder that had a snake that was sooo nervous that it would get sick from being handled to much due to stress. Although this is extremely uncommon it shows that they are individuals and they all react differenly. I have over 30 snakes and one gets an RI and the others don't. They are all in the same conditions but some aren't as strong as others or one is more stressed for whatever reason. When you see signs of an RI it could have been brewing for days or weeks before you notice. Thats why it is recommened to keep them separate because by the time you see evidence it has been going on for a while.

I just got over bronchitis, it started with a sore throat, then sinus congestin then full blown flu/bronchitis. Germs incubate in there host for some time untill the full blown illness is noticed. Now my wife never got bronchitis(just a cold) but she very easily could have because she was exposed to it.

Hopefully you other snake never catches the RI but he the odds are much higher if he has been in contact.