To match levels , because the louder one almost always sounds
better [ or stronger ] if you have two mics or Y'd it
put one channel out of phase and mono them changing one fader
until they null as much as possible ,
phase differences may prevent this from happening completely
but you should get close .
Also if you have the time , recording it on something so that you
take away the real time elusion of change [ or difference ] ,
from the equation [ you know , your eyes ]
Even better and if you can only do one at a time , recording a repeating
phrase and punching in without looking where is interesting on playback .
Good ear education !