When "User A" in "Forest 1" schedules a meeting on behalf of "User B" of "Forest 2". What you are doing is scheduling a meeting on behalf of the "User B" stub object that resides in "Forest 1"
Collaboration Services is only a one way sync. It sync's from "Real Users Mailbox" --> "Stub User Mailbox" in the remote forest. It will not sync from the stub user mailbox back to the original user mailbox.
If "User A" in "Forest 1" convenes a meeting on behalf of "User B" of "Forest 1" that meeting would be synced to the stub mailboxes in Forest 2.
