Combining Ashura Mode (x3.5 Nin) and Sage Mode (x2.8 Nin) gives a total of x3.6 Nin in this game. xD
Maybe when combining boosts the formula could work like this.
A= Boost #1
B = Boost #2
A x (B/2)
This idea states that the first boost you activate gives it's normal buff, but the second boost that you're adding onto the first will only give a half boost. Like say with Ashura Mode and Sage Mode.
Ashura Mode = 3.5
Sage Mode = 2.8
3.5 x (2.8/2) = 4.9
Divive the second boost by two and then multiply it by the first boost. Simple and fair. =D