So as a Dancer, with Behemoth +1 and Joyeuse, your base attacks per round (before double attack and joyeuse activation) would be 2, with double attack it would be 2.1 (since you have 2 hands, correct?).
So after 100 attack rounds, you would attack 200 times, with 10 double attacks, meaning joyeuse could activate on any of 190 attack rounds, except you're only holding it in one hand, so it could activate in an 50% of 95 rounds, ergo dual weilding with Behemoth Knife doesn't affect Joyeuse at all right? And you would end up with a total of 257.5 attacks per 100 rounds?