Firstly, I have been known to forget to feed them, giving them a head start in starvation. When I do remember, F, the biggest one, scoffs all the food. B, the smaller one, floats around the bottom and rarely ever comes up for any flakes. However, if you continue to watch for another couple of minutes, F will regurgitate some, and B will snaffle a couple of bits of the smaller, ready processed vomit when he thinks your back is turned.
I think they're actually better at it than I am.