Yes, you'll need another heat source. Period. The flexwatt does provide belly heat, but not much more, especially in an enclosure the size of a T8 (or even half of a T8, for that matter). In a 15qt tub, it's a different story. Here are the three options I would consider, if I were you:

1. 80w RHP (radiant heat panel). If you are using one open T8, this is the way to go.
2. 2 - 40w RHP's. If you are getting the divider with your T8, so your BP can start out in a portion of the T8 (you can move the divider as your snake grows - remember, smaller and/or more cluttered is better for bp's), then you can install a 40w rhp on either side of the divider. The plus side of this, if you're only using one side of the T8, you can just order one T8 for now, and get the other when you need it.
3. Get a heater and heat the room the snake is in to high 70's-mid 80's. In that environment, your flexwatt would probably be sufficient. Not ideal for most people.

LED's are great for show, but don't expect them to raise your temps at all. They don't put off enough heat to be considered for anything when it comes to heating. Some of the older fluorescent light fixtures would put off a tad bit of heat, and in a warm room, they could raise ambients a degree or two, but since they need to be off at night, it's not a good heat source to be dependent on.

Another solution could be ceramic heat emitters. These are a good source of heat, but would require you having a hole cut in the top of your T8 and this will make you go nuts getting your humidity right. I don't recommend it, but it is a solution that some use.

Now... thermostats... Like you said, if you want to run flexwatt and an RHP, you're going to need two thermostats. I run this same setup with a Herpstat 2 and could not be happier with it. Once my bp gets a little larger, I will most likely remove the divider in my T8, stop using the flexwatt, and just have my Herpstat 2 control my two 40w rhp's. One on the warm side, and one on the cooler side. The Herpstat line is expensive, but well worth it. I went with a cheap solution at first, and ended up coming back around and getting a Herpstat. I should have saved myself $50 and just went with it upfront. Since I got it, I have no worries.

Hope this helps.