1st question – Does the bay window get direct sunlight especially in the summer? If so I’d start by putting a small tank there setup like you’d have the bigger tank and put a thermometer in it. You want to see how hot it gets with direct sunlight. It might be way to hot for a snake.

2nd question – Have you quarantined the new ball python? Before you start putting them together make sure to go through a 90 day quarantine period.

As for the tank idea I’m not really sure how to do it. It would make more sense to me to get two smaller sized tanks (20L maybe) and put them side by side. Certainly from a cleaning perspective 2 smaller tanks are easier to handle than one large. Try looking on Craig’s List for used aquariums. Often people will get rid of them because they don’t hold water which is perfect for reptile owners.