Its weird, its both the same game just one is standelone and the other is the same game included in ED/Horizons. Do they not want these two groups to talk with eacht other on the Forums and thus the seperation? ^^
They'll merge eventually, for now though I'd guess that they are having them separate so that any new customers coming to the forums immediately see "Arena" and come here, while players looking for their CQC forum don't go "HEY! Where did my subforum go!?" This forum is also better suited to threads about "what's the difference between these two" that will be around for the next week while this all settles down.