Regarding using ldap across 2 domains, I dont think that can be done unless the two domains are part of the same schema. As I understand openfire, you can only query one LDAP schema per installation.
There is a good walkthrough about how to link two xmpp servers here.
Very simple to do actually, whether the xmpp servers are openfire or googletalk or whatever. Just keep in mind, that for intranet/extranet situations, you need to make sure that the value set in “Server Name” and in Sytem Properties —>
xmpp.domain must resolve to the internal ip of the server. If the server is set for internet and intranet access, then split dns needs to be set up. We lost some time on that one. . .