What JomSocial describes will cause you lots of problems. Facebook introduced "App Scoped User Ids" back in May 2014 with the Graph API v2.0. What this means is that each Application will return a unique ID for each user. If you try to use multiple applications on both domains, each one will see a different ID for the user and JFBConnect won't be able to log them in. On May 1st 2015, App Scoped User Ids will be a requirement, though your app may already being using them if you created your app within the last year.
You can read more in their FAQ about how it's possible to re-map users from one app to the other, but it's not something JFBConnect supports and isn't something we're planning to add:
developers.facebook.com/docs/apps/faq#how-to-map-users
So, up until a year ago, we would have been able to recommend a similar thing to what JomSocial recommended (and we did do that for some users who insisted). Now, we wouldn't recommend doing that at all.
Also, Facebook isn't the only social network with App Scoped IDs. LinkedIn and another I can't remember right now do it as well. Those other networks have *no* method to map users across apps.
My only suggestion would be to use a language suffix, like:
nl.zooport.com
en.zooport.com
All social network apps can work on multiple sub-domains, which would do what you're looking for.
I hope that helps answer your question, even though it's not what you were looking to hear.
Alex
Using 2 language domains, but one user database
. I would like to use two language domains though. Zooport.nl for dutch and zooport.co for english speaking people. Everything is the same except for the language file.
