Coherence performance is related to the latency between nodes. Having one cluster spread over 2 data centers could harm performance (some timeouts could have to be changed to prevent nodes from data center A to claim another node in datacenter B is out of reach/possibly dead).
When you lose network connectivity between the 2 data centers (note i'm not saying "if you lose connectivity". It WILL happen), you're welcome into the "split brain world", each half of the grid believing the other is dead and claiming to be the "master". And thus, if you have data replicated on N nodes, the master/backups are redispatched all over each datacenter, harming performance for a few minutes (the timing depending of course on many parameters...). And of course the data will no longer be synchronized between the 2 data centers. The quorum has to be thought of, and stuff like that...
I might be wrong, but AFAIK I'd rather have 2 separate clusters. I believe 12.1 has new features to replicate data the the master grid to the DR one, I have not been through all the new documentation.