| Cult of Vorg |
Depends on what you mean by stack..
Sohei weapon training is as the fighter class feature, so a sohei/fighter could take different groups and be able to flurry with all of them, but taking the same group twice wouldn't stack the bonuses (same source, overlapping weapons from different groups only get one of the bonuses), and would not trigger the level-based increase to an existing group.