I would honestly forget about the lights and flexwatt , get the right sized RHP, and have animal plastics cut a spot for the flexwatt just in case you do need to get it in the future. An RHP should be sufficient enough with the ambient temps you said your room would get to. Flexwatt and the florescent lights wouldn't be enough to keep the ambient temps high enough in a t8. I would also suggest not having two snakes in a split t8. My big guy is only 1800 grams right now and when I see him on one side of the cage, I can see that half of a cage would not be big enough for him.
Also, depending on what size snake you get, you might not need a divider. I put my female in a full t8 with plenty if clutter when she was around 500 grams. She has been absolutely fine in it.
The led strip is definitely worth it. If you can u would just by it now before the cages come because it is way easier to install when you are building the cages.
This pic is from the first night I got the cages and was still setting them up. The led strip I got from Lowes. Really glad I did.