When i asked (as i really didn't want to go through and modify a ton of apps), i was told it was cumulative. I'm still asking if this will be patched... as its a hokey work around, but slightly less so than killing off services .
To my knowledge all snapvol.cfg's of the appstacks are being merged together (as sabian says here) and if you have 1 appstack that every user gets it would be enough to change the snapvol.cfg on that one appstack.
Regarding the order in which you assign appstacks. I believe (not 100% sure) that indeed the last one assigned is being attached the last and thus "winning" in regards of applications installed within an appstack. Let's say one appstack has Chrome 50 and the other Chrome 60 (yes I know, very old but just for the reference
) the one that is being assigned last will win because both applications are in the same location.