I often use alternative tunings for 6-string guitars. Now I'm facing a 12-string guitar.
Do alternative tunings exist for 12-string guitars?
Traditionally a Twelve String guitar is tuned using the same pitches as a 6 string guitar except the "bottom" four pairs of strings (heavier strings) are tuned an octave apart and the "top" two pairs (plain steel treble strings which are actually at the bottom if you are holding the guitar upright) are tuned in unison.
A set of strings for a 12 string will include the proper gauge strings to allow for the bass pairs to be tuned an octave apart with relatively equal tension.
So for any alternative tuning, you would simply tune each pair of strings to the same notes as you would on a 6 string using that same alternate tuning. Except that the smaller of the two strings in the bottom four pairs (the wound bass strings) would be tuned exactly one octave higher.
Many folks have contemplated tuning the pairs using some other formula besides octaves (such as thirds or 5ths). But this will not work out well with many chord voicings. Although it might work if you are just playing individual notes one at a time.
Exactly the same options as for a 6-string... so long as you keep the pairs matched.
[right now I'm imagining what it would be like to have one of the pairs out by a semitone ;-)