The only unthreading problem I've ever known of (and not on a tandem) is having an Italian or French threaded BB. These have a right threaded right side BB cup and thus are very prone to unthreading. Captain/Stoker/Single BB cups have basically the same likelihood of unthreading and the same general forces applying to them, so just because it is a tandem doesn't make this more/less likely as far as I can figure. If this is English threaded (left threading on the right cup) then it shouldn't unthread unless you spend a lot of time pedaling backwards, and even then it should stay put since there isn't much force working against you.

An easy first fix, no matter what the threading of the BB is would be to apply some locktite to the threads before installing. This might provide just enough resistance to slipping to hold it all together.