Replicating JDBC proxy

I would like to have a constantly updated copy of my DB. In order to
realize this I have though of the following scheme:

A server is equiped with 3 NICs and is running a special 'proxy' JDBC
implementation.
The only thing this JDBC proxy does is:

1) Forwarding every update request on both ports. On each port a
server is running a JDBC compliant DB (in my case, postgresql)

2) Forwarding every select request on only 1 port.

Theoretically, this would give me a total synchronized DB, wouldn't
it? However, this sounds relative simple to implement. I've seen a
couple of replication efforts undertaken for postgresql that seems to
take forever to finish.

Am I overlooking something?