Zack, The reason I don't keep this combination is because they may compete for the same spawning sites. Both might use the shells. In this case, If there are enough spawning sites, its likely to work though. It's the same reason that I don't like keeping breeding pairs of plecos in with my apistos....they end up competing for the same cave or coconut shell. Sometimes I have even found that although I offer multiple spawning sites, the apistos get bullied out of their cave by the plecos although there are others available. Nobody gets hurt but one species or both don't spawn well.
Punks are called semi-shelldwellers. they are normally spawn in crevices between and under rocks (or other decorations)....they'll also use shells if that's easier. I keep my punks with other rock dwellers and offer them shells so that they have options. Here, I suspect it'll be the opposite. They may take the rocks for spawning and leave the shells to the multis.
Ernie, are you seeing the punks tending to choose shells or rocks in your tank?