Confusingly, Couchbase Server has its own replication protocol called XDCR, which you can read about in the server docs. It’s not really designed for the same use cases though; in particular it doesn’t handle conflicts, so you can’t safely use it if docs are changing on both sides. It gets used for things like mirroring a master cluster to a slave one in a remote data center to increase availability.
There is an experimental tool called SGReplicate written by @traun, which lets two Sync Gateways replicate with each other. That would do what you want, I think. You can find it on Github: https://github.com/couchbaselabs/sg-replicate