^Actually the good thing about recording is that you can pan the instruments. Panning both guitars reasonablyto either side would enable the listener to be able to hear both guitars, without destroying the mix (or his/her ears).
When jamming with two guitars, and both have the same EQ and channel... it'll be a problem. Both guitarists will probably have to compromise their tone and gain. It shouldn't really be a problem if both guitars are in different channels - but make sure that their volumes are not overboard.
If both guitars are in the same channel, first of all it depends on what each person is playing. If both are playing the same chords I guess it's obvious you can't hear the other - it sounds the same. If one is playing a lead or appregio while the other is playing rhythm chords, the former should be a little louder. If both are in distortion, the lead should have more mids and trebles to slice through the rest, while the rhythm should have more bass - to keep the rhythm.
I don't know what other problem combinations there are.