So far, I've just been recording rythym guitar tracks hard left and right with the same guitar
So you are telling me, Sonic, that you have one make of guitar recorded on one side and another different guitar make on the othere side?
If you do this, do match double-coil pick-ups for both sides (Gibson Les Paul left, Gibson SG right) or can you have double-coil one side (Les Paul) and single coil (Strat) on the otherside playing the same rythym part?
No, the same guitar is ok. But you can use different guitars, especially when quad tracking - do 2 tracks with one guitar & amp, and 2 tracks with other gear. Then blend them to taste.