Writing to two masters is like walking around with a loaded shotgun. Eventually one of your instances will fail and when it does, replications position & synchronization information could easily become corrupt!
Reason 8 – Crashed nodes cause big problems
MySQL instances, unfortunately can crash. When that happens, they don’t always sync the replication position properly. That’s a big risk. It’s one of the reasons why operational DBAs know that replication slaves need to be rebuilt from time to time.
If the replication position doesn’t sync properly when MySQL restarts it may do so at the wrong position and replay or miss some transactions. What this would do to an active-active setup is unpredictable. Again no single authoritative master means trouble!
MySQL 5.5 has introduced some parameters to address this. During my tests I’ve seen a performance hit, so use them with caution.