What do you think about the words simultaneity, simultaneousness, synchronicity?
I personally think that all derivative forms of simultaneous should do the job just fine because first of all it's the closest one in meaning and secondly it really is the only word that's completely synonymous with concurrency.
Alternatively, you could try using different phraseology altogether. What I mean by that exactly is that instead of alternating the words concurrency and, let's say, simultaneity, you could use phrases like happening simultaneously, happening in sync, concurrent events, running in parallel, simultaneous computation, et cetera. There really are lots of possibilities.