this is a really interesting idea. My daughter and I want to get a second BP but we're pretty sure mom isn't going to agree to having a second cage set up. So I was reading the VPI care sheet and it recommends between 400-600 square inchs of floor space for an adult.

A 20L aquarium dimensions are 30"x12" for 360 square inches of floor space. A T8 divided in half is 24"x24" for 576 square inches of floor space. Even a T5 divided in half is 18"x24" for 432 square inches of floor space. The care does go on to say that most adults will be very happy in a 20L tank.

Im guessing there will be slightly more effort in heating and providing a temperature gradient. A T8 seems like a good option to separately house two snakes in one enclosure.