For just the selected information you want something like postgres BDR will do it. It doesn't matter if the information is slightly stale I imagine.
Postgres-XL is a little different more for scaling a single cluster, I think in this case, it is two databases with different information.
Base_synchro is a waste of time IMO. So many things it doesn't support and so many other modules that break it. Like deleting a record, or making more than 2 servers play nicely. Or installing base partner merge or product merge. And it is hell slow. Note while my name has vanished from the credits in the port to 8.0, I wrote a lot of stuff for that module in v7.0. Honestly I wouldn't waste my time with it, it can be made to work, but the ongoing admin effort for anything but the simplest case is a nightmare, the best solution in Odoo is connector. Especially since you can control precisely what goes where.