As far as I know, you cannot cross ref any parents. Only Parent/Child.
You can cross ref a parent/child relationship whether it is in the same sheet or different sheets.
Ace User since 1998
I believe you can link them using the wdalttag attribute. Not sure, but I seem to remember doing that several years ago.
i have drawings with duplicate parents (peer to peer) on the same sheet cross referencing, although I am currently having issues with it. One thing you may want to check, in Project Properties / Cross-Reference Options, ensure Peer to Peer is ticked. Im not sure if this is necessary, but its worth a try.