Hasta La Motion!

maybe this is a stupid question, but i can’t for the life of me figure out how to do it 🙂

let’s say i bring in an object/objects into nuke from a 3d-app and then create additional fx-elements in nuke…think water-splashes on cards for example.
or even more simple…different scanlinerender-outputs in nuke and i want to use the 2 as holdouts for each other.

is there a way to use one as the holdout for the other so that it’s "matted out" basically!?

i tried using zmerge but that doesn’t do the same thing. also i’ve gone through various layermodes in combination with zmerge…but that all lead me nowhere it seems 🙂

thanks in advance for any help!

cheers
chris