Throwing in late -- I've had recordings come in with absolutely terrible phase cancellation in the vocal... I always asked about if there was a non-wet mult somewhere (usually an aux send that wasn't 100% wet). That's the problem the majority of the time.
More than a few times though, the engineer would swear up & down that there wasn't anything like that going on, but at some point he'd say "I wonder if it was one of the microphones..."
I'd say "ONE of? You used more than one?"
He'd say "Yeah - One right up close and another (a few inches, a foot, whatever) back a bit."
Super-bad idea. SUPER bad.
Even if you make a PROPER "stereo" close-up vocal recording (as in, XY, ORTF, MS, Jecklin, etc.) the *tiniest* deflection of the source is going to cause absolute chaos in the imaging.
You can record as many sources as you want, pan them however you want. But a single source is a single source. "Stereo" is the difference between left and right. As Jay mentioned, if you want to capture the room (usually completely separate from the source - a stereo pair distant from the vocalist) along with the source, that's fine. But if you're trying to get some lush, wide vocal spread by multi-mic'ing a single vocal, it probably ain't gonna happen.