possible to have 2 Time machine "silos" on same machine?

[clay]

no, it's actually a question that comes out of having 2 different-capacity extra drives lying around that would work individually with Disk A and Disk B, but since they're not the same size and aren't big enough on their own to backup both Disk A and Disk B, I'm trying to see if they'd work separately, since that's the only way I could think of to fully utilize both extra disks.

(how's that for a run-on sentence? :-) )