federated tables? synchronization? replication? [mysql]
You are asking for pain by having the data split among machines is a haphazard way.

Consider Galera (PXC, etc) as a multi-master approach.  If one node goes down or is isolated by a network outage, it is self healing after things come back.

Yes there are multiple solutions (now) for multi-sourcing a slave.  MariaDB has had an offering for some time.  Oracle's MySQL has one.  Galera allows each node to be slave to some 'external' Master.