look at it like this
if u play 1 tune as loud as u can on ur mixer it should be below or on the redline light
if u mix in another tune in fully eq'd, chances are its gonna red line, distort and sound shit.
u need to subtractivley eq each tune so it doesn't gain so much volume and distort.
a good trick is to never have 2 bass lines mixing with both bass eq's up full.
having one bass eq on full and the other on min will sound cleaner. (but not always the best way to do it)
with treble.... most of the time i like to have one down just a notch or too, this saves ur high end sounding really tinny and annoylingly high pitched, with a million hi hats going crazy all over the place.
just mix 2 sets of drums and have a play about till it sounds good. try recording the same mix several times e'ing it different each time and listen to the difference. 7 times out of 10 i will have 1 trebble eq at 12 oclock and the other at 9/10 ish.. but each tune/mix is different.
mid ... i use this as the volume (kinda) if you have ur bass and trebble eq'd nicley u can use the mid as ur master level for the track, i dont use the volume fader at all. if the bass and trebble is eq'd low then the mid volume would have decreased slightly anyway.
hope that sort of helps, nothing is rule!! and the above is just my views .. if it sounds good do it. remember .... less is more!
tip...if ur gonna subtractivley eq the tune ur bringing in .. u need to get the eq's back to 12 o clock before u cut the other tune away.. it can be tricky to get right!