I could be wrong but it seems like MongoDB isn't really partition tolerant since it relies on replication or sharding (just like an RDMS). I think it is more of an AC (with a caveat on the C)
As you say, MongoDB has support for replica pairs that are partition tolerant, but part of this functionality is in client libs. For Ruby, you register both dual masters in a replica pair with the client library. This seems like a practical approach to me, or at least fairly easy to work with.
