You mean running the BOINC client on at least one of the GTX 260's as well as GPU folding? Both of them will be fighting for GPU resources and both should suffer. Or worse yet, Nvidia drivers don't support something like that and both client crash (or one of them does).
Why speculate? Test it?
If you are not into S@H for points, one way you can do is to configure S@H to use CPU only, then given enough cores the clients should happily co-exist.
The Model M is not for the faint of heart. You either like them or hate them.
Gerbils unite! Fold for UnitedGerbilNation, team 2630.