This is a common problem for newbies and veterans alike, PHASING! Its basically what results when you put more than one microphone on a single sound source. The sound traveling to each microphone arrives at the same time essentially, but a different part of the sound wave is picked up by each mic. When this happens you get exactly the opposite of what you intended. Instead of this big cool sound of two or more awesome mics, you get a thin, nasally, metallic sound.
There are definitely ways around it. One of the easiest ways, but takes another person, is to put up one of the microphones, and while placing the other microphone(s), listen with headphones. You will definitely hear phasing going on. It will sound exactly like a phaser guitar pedal as you move the second microphone. It doesn’t matter if you are mic’ing a guitar or an amp or a snare drum. It’s physics baby.
There is another type of phasing that occurs with outboard gear and cabling too. Check out this page on what phasing is and how it occurs. LINK
Check out this video on how to fix it once you have it recorded. This also is an aid to know what phasing is and how to stop it before you get it recorded.