You can think of 합을 맞춰 as 서로 맞춰서 (in sync/choreography with someone). Using 합 this way seems to be a recent thing. It seems to mean something like "jamming" in music. Normally, 합 just means a sum (합이 얼마야? = What's the total?. 합을 내다 = Take the sum, 합하다 = add up). 춤 동아리 누나와 합을 맞춰 추는 춤이 었는데 동선이 엉키며 부딪쳤다. = It was a dance I did with an older sis in the dance circle, but we had a collision in a foul-up of a move.