It's a vailid idea, but whether it will be of any good or bad in any particular situation is a coin toss. Matching mics to voices is a bit like matching shoes to feet; what works best for one voice isn't necessarily best for another. The best you can do is experiment around.
I've seen film of Sam and Dave using dual mics, one on a boom stand like normal, and the other just taped to the top (or bottom) of it. I couldn't tell you the models offhand, though.
Also, another trick is to use a second mic to fatten up thin vocals by pointing the second mic down toward the chest.
G.