no they're not interchangeable actually. "in half" would be used as more of an adverb. examples: I cut the apple in half for you ("cut" is the main verb, "the apple" is the direct object for that verb, "in half" is the adverb/adverbial describing how it was cut. / I work much closer to home and my travel time was cut in half "in halves" or "halves" is a plural noun. the singular form would be "half" Examples: I picked up the two halves of the log that i split for fire wood / I cut the apple in half so i'll take one of the halves and you can have the other half.

half is singular so there would be 2 pieces in halves is plural so there could be 2 pieces or 4 pieces, so it's using fractions 1/2, 3 1/2 or 0.5, 3.5