You can do something like this with resource share; if you set the primary project(s) to a high value, and your backup project(s) to a low value you will get something like what you want. The way it works, is that it splits up your computation time using the resource share factor... If your primary project doesn't have work very often (like mine) then whenever it has work, it will run most of the time to make up for time that it lost to other projects that had work. An example of my distribution follows (although I have it temporarily suspended to hunt badges at WCG)
BURP 1000
Renderfarm.fi 100
Enigma 10
WCG 10
ViP 1
Because Enigma and WCG are the only projects that regularly have work, they build up a lot of CPU time. Thus, when BURP and Renderfarm.fi have work, they will run most of the time to make up for the time "debt" created when the other projects had lots of run time. This also works for ViP, even though it has a very low resource share.