Mysql master master with high latency
PXC (and other Galera solutions) require one network hop at COMMIT time.  If your application can combine writes into a single transaction, that will help at some level.  However, still each COMMIT will take 300ms (or whatever it is).

Regular MySQL can do Master-Master and have both writable.  That replication is asynchronous.  However, there are a number of caveats.  Auto_increments should be configured with auto_increment_offset and _increment.  UNIQUE indexes are problematic; the app must figure out how to avoid 'simultaneous' insertion of the same UNIQUE value on both continents.  Note that replication will simply stop until you manually do something.

Another possibility is NDB Cluster.  However, that has a lot of differences in the syntax and functionality.  The advantage is that it is "eventually consistent".  You set up rules about what to do if, say, duplicate keys are inserted.

There's an old saying:  You can have any of X or Y or Z; pick 2.  So, I ask, what are your 2 reasons for wanting multi-Master?  And what 3rd item are you willing to sacrifice.  (It sounds like latency is not #3.)