My tube amp head has 2 inputs, a high gain and a low gain. Can I plug 2 guitars into the amp, one in the high and one in the low, or would that damage it?
No but if both are played at the same time, it will sound like crap. It works fine in teaching situations when both are not playing at the same time, but no. I doubt any damage would occur, except to your sense of enjoyment.
^ Exactly. Theoretically, for it to work well, you'd have to be exactly replicating the signal source into both channels so that the sound is processed the exact same way and the speaker/driver is moving in the exact same direction.