There are many reasons that we do not house our boas together. In the wild, boas are solitary creatures and are not social by nature, If there are more than one in a single enclosure, feeding time can become a real challenge (They will eat one another). It is much easier to keep track of when a boa defecates or, if not well, regurgitates when they are housed by themselves. One boa will tend to be dominant and monopolize the warm spot leaving the other to live in less than ideal conditions which could lead to things like a respiratory infection.
It is my experience that if a boa is calm as a juvenile that it will remain so as an adult, but I would suggest maintaining the same level of human contact that they get when younger. Females that have just given birth can be a little testy, but then again so are humans!?! LOL
Pink or white belly? Well it is the same as green or brown eyes or hair color. It is simply their genetics that determine these things.
Hope that this is the info you were looking for.
Good luck.
Denny 