For starters, do NOT house them in the same room for atleast a period of 6months to a year. This is to quarantine them from from the possible spread of IBD, mites, etc. IBD is very, very bad and can/will be spread between your snakes if your RTB happens to have it. Even after that time period, I personally would not house the animals next to each other just to be safe. So this means your going to need another basking lamp.

Next, ultimately they you're snakes so you can handle them however you want, however I find that having both my bp and RTB out at the same time just seems to stress out my ball and is something I avoid.

Yes, a full grown RTB can live in a 55gal tank, however RTBs have been known to break through the glass because of how hard they strike. I would look into getting either a boaphile or a reptile plastics tub for you boa once it gets some size on it.