Mites can be an issue at expos. I don't think the problem is so much the breeders as the re-sellers, but I really shouldn't speculate.
I think there is less of an issue with separate hinged doors. But, to be perfectly frank, newly acquired animals should be kept in different rooms altogether or at least far apart. With only two snakes, it's less of a concern, especially if you buy them both from the same breeder. Make your own call.
I say stick with the divided T8. You want a hatchling? Well the divided T8 will serve you life long. You can start in just the half. When ready you can move them up to the full. Plus a divided enclosure will give you flexibility in the future.
Since nobody else has chimed in, substrate. Expect to need to experiment with this before you pick up your first noodle. PVC enclosures hold humidity really well. I use aspen and find that I can regulate humidity with water bowl placement. I tried coconut (Eco-Earth) and Forrest floor, but it was WAY too humid. Within hours it was practically raining inside the enclosure. For a few days, during a shed cycle, that'd be great. But everyday? No, you'd be asking for an RI or scale rot. But you may find things are different in your area. Try different ones for a few days before hand and see what works.