It should stack with Offense Up.
That being said, he wouldn't necessarily do double damage with 100% Offense. If he has other sources of offense on the team, he might be going from +75% (random example) to +175%, which would be 2.75/1.75 = 1.57 or only a 57% increase in damage. So I don't know exactly how it would interact with his stacking Mastery, say. There could be numerous other interactions that could be reducing damage.
The best you could do is to take several videos of having neither bonus, both bonuses, and one or the other, and compare all possible factors between both situations to see if it seems to be working.
