This plugin lets you control filter frequencies using midi input, so you can dynamically duck an instrument across an entire song. I'm using the midi of the lead guitar to carve holes in the rhythm guitar so it will fit better in the mono mix:
You can see I'm making holes for the fundamental and the first four harmonics, maybe I should go up even higher. It works for chords as well so that's pretty cool, I've seen dynamic eq but I haven't seen anything that lets you control it with midi. You should obviously try to pick guitar tones that fit together to begin with, but this technique allows you to turn the rhythm guitar up way higher without ever masking the lead