PANNING
is purely subjective.
ideally, depending entirely on the arrangement of the instruments and the song,
they should be 'complementary'
meaning, multitracking rhythm guitars just for the sake of multitracking, may be clownphuckery
i double track a lot...
listen to any of my tunes, and you'll probably hear two entirely unique rhythm parts panned left and right,
but whereas i used to go whole hog, 100% left and right,
i've found that the sound i like best today,
is where i bring them in just slightly..
maybe, 85% left/right...
hard panning may sound awesome in a huge room with killer speakers and uber-headroom power,
but most folks listen close, or with cans or ear buds, and 100% panning is just a bit too extreme.
classic rock,
say, deep purple's 'highway star'.....
LCR mixing mostly, it seems, guitar hard left, keys hard right...
they mix in a bit of reverb on both, but pan the reverb to the opposite side of the instrument,
makes it sound tied together mix wise, and still clear and separate.
there are so many options...
bottom line is, you don't want one side heavier than the other,
or have 'holes' in your soundstage while mixing,
unless it's specific to the song or arrangement.
listen to MASTODON, dual guitar players, see how they pan multitrack..
dream theatre...
joe satriani..
Queen, with all of may's multitracking...
there's lots to learn by just putting on a good set of headphones, and picking some classic mixes apart.