UPDATE: After queueing a couple blitzes together on preservation evoker and havoc dh, my friend and I attempted to queue together on balance and preservation. We were unable to queue together.
We then opted to try queueing together with my resto druid and his havoc DH. Lo and behold, we were able to! I looked at our CR, and our CR was exactly the same. It is possible this issue is directly related to CR or MMR. Or it may have nothing to do at all with any of the things I am testing. Will update this thread with more info.