Best RB duos tend to degrade into ______ - amazing all-time guy and whoever his backup happened to be. You'd take Barry Sanders and Player X over Chubb and Michel, because it would make sense to.
The better method would simply see who was the best/most productive backup RB ever, and go with that. Some off the top of my head:
Bo Jackson/Fulwood, mid-80s Auburn
Neal Anderson/John L. Williams, mid-80s Florida
TJ Duckett and whoever the other guy was - MSU late 90s
McFadden and Felix Jones at Arkansas (yes, Jones' ypc would decrease if given more carries, no matter what "type" they were)
Any number of Wisconsin duos/trios, a la Fletcher and Moss