This should work if you use the global catalog and then use the root of your forest for the base dn. to limit the users you want to display… you’ll need to edit your group search and user search ldap query. I suggest using group membership to do that…here is a post I wrote up a while ago…with a few tweaks, it should work with child domains of the same forest.
Another options, would be as wroot stated. spin up two servers, one for each child domain (separate xmpp domains), and then use S2S to federate between the two…