Well it's setup to follow the leader and assist problem is it stops following for some reason or another after awhile, and I can just follow that is great but then eventually your other toon will time out and get DC'd for inactivity.
I would almost bet has to be some programming bug for it to not follow and assist consistently, and if your having them both loot it's even faster like HB2 uses the in game follow and when the Grind Bot gets outta range the combat assist bot gives some type of not in range bug....