did the snake recently regurgitate the other one? or was that a while ago? the reason I'm asking is because if a snake regurges it's often best to skip a feeding and then start it back on food with a smaller than normal meal. Supposedly when a snake regurgitates it depletes much of the beneficial bacteria in it's digestive system so waiting before feeding again will give it time to build it back up. If you feed it again too soon sometimes this can cause a relapse of regurgitation due to lack of the levels of bacteria needed for proper digestion... leading to multiple chain regurgitations and a sick snake. I am sure a shot of nutri-bac or whatever products they have out there for beneficial bacteria would not hurt also.
If it's not that I don't really have another speculation... could be just a freak thing... I don't keep boas so I don't know off hand the requirements for proper digestion in BRB's. My only suggestion would be maybe being fed outside of the cage is more stressful than you thought and is causing this to happen. I don't feed any of my snakes outside of their enclosures for that reason. I just put down a paper plate or damp paper towel and stand by as they eat.