AFAIK, they stack, so no secondary effects cancelation when combining them
Oh they stack, its just that we hit the accuracy 'cap' (dont get pedantic) with less than those buffs so the extra is 'wasted' 
There are reasons for alternating how you use them but both together is still yum-yum. 
ok then. I found the site where I read it here, so you people don't think I'm just making stuff up. And in looking for it, I found plenty more places that said they DO in fact stack aswell.
I could be wrong on this, but I thought that when used in combination build up and aim cancel out their secondary effects.
i.e. build up: +100% damage, +50% accuracy
aim: +100% accuracy, +50% damage
but used together: +100% damage, +100% accuracy.
So maybe you'd get more mileage alternating between them.
*I read this on a forum somewhere so it could easily be wrong*