That sounds a little too easy and 'hand-holding'. There'd basically be no penalty to improper timing.

One thing they could do, is allow buffs like that to form stacks, but have successive stacks have diminishing returns.
1 stack of Trick attack gives a 10% debuff.
2 stacks of Trick attack give a 15% debuff.

This way, it certainly wouldn't be worth having two Ninja's, but if you did, and their buffs overlapped, it wouldn't be wasted, but there's still incentive to time them apart.