I would like to add something helpful to the particular issue, but can't, since my character is MA and I only happen to see duelist animations performed by Duelist mission NPCs.
All I can say is that this problem isn't limited to Dual Pistols and affects pretty much grab as a whole. I'm not so sure about hit-miss rounds, it might even be that they work correctly.
However, hit-hit combinations and also finishers are very often bugged -- so that one or (the mathematical sense of or" ") ) the other opponent freezes while the other one performs his move.
Although MA grab attacks work correctly quite often, I've noticed that hit-hit grab rounds with Duelists are bugged more often -- or if I think about it, even always... I'll just pay more attention next time.