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I read a book a long time ago that I'd like to revisit.
Main plot involves a protagonist that learns every time he dreams he's actually living a dual life in a monastery. Every dual person who sleeps in one world wakes up in another and has dual magic. Life/death or knowledge/ignorance or fire/water. Protagonist chooses life/death and one his hands is dead as a result. He realizes the language he studied in what he thought were his dreams is real and can speak them to other magicians that live in the two worlds.
I remember that if someone died in one world, they would lose their magic and have to live out the rest of the other life in the other world.
If I had to guess I read it about 20 years ago, though I do not know the original publication date. I have a vague memory of the cover being of one or more people underground. If memory serves the fire/water magic worked where if the caster summoned a lot of fire then water would also appear to 'balance the scales'. There was a scene where the fire caster summoned fire in an underground tunnel and a nearby reservoir tried to flood the tunnel to compensate.