Problem in MySQL cluster when using two clusters
I'm assuming from the diagram that this is not in fact MySQL Cluster (NDB) but rather the informal sense of the word "cluster."

This should be fine as long as you don't intend to write to M2 or S2, or if you do, that the writes are exclusively to schemas and tables that do not exist on M1.

There are three probable causes:


You didn't enable log_slave_updates on both M1 and M2.  Without this option, incoming events don't propagate outward.
You haven't given each server a unique server_id.
You're doing replication filtering (rarely a good idea) and its configuration is being interpreted differently than you expect.