I'm confused.

For example if you use both Jinpachi and Edo Sasori on Kushimaru/Deidara he will still only do 2 attacks

This right here is self explanatory. But then you go to say

Jinpachi + Killer Bee + Edo Sasori will have 4x attack result

So if Jinpachi and Edo Sasori don't stack on their standard, how the hell does it attack 4 times? Shouldn't it be 3?