If you use something along the lines of a shotgun and an omni mic, you'll get very different sounding results from each.
The other use I can imagine is a system I sometimes use for ADR with two shotgun mics on stands so I can get a close perspective and a further perspective, it also means if your closer mic clips theres a chance the performance isn't lost as the second one was less likely to clip from a distance.
Sometimes for ambient recordings ill use two mics on a mic stand with a stereo bar, and walk that around with a wind shield over the top, into my Fostex-LE.
Recently though I have found it easier just to concentrate on getting great sound on one mic, it saves me space on my recorder, and stops me getting overwhelmed by having to adjust gain/monitor to different mics at once.
I heard on the Kings Speech the sound recordist put microphones almost everywhere so he had loads of takes he could choose from.
Everything is worth a try though!