MongoDB automatic failover between 2 data centers
First, you are correct. If the replica set doesn't have a quorum, it will relegate to secondary state.

The solution is pretty easy: put two data bearing nodes in each of the two datacenters and an arbiter on a cheapo VM in a third datacenter. Hell, a free AWS instance should do the trick. It is very unlikely that you'll loose two of three datacenters at the same time.

From my experience, everything else will fire back on you.