Could you use a "coordination" table (or even a separate .txt or .ini file), that would be updated by whichever mIRC makes changes, to record that changes have been made (and which tables were updated, if necessary)? The other instances of mIRC could monitor that table/file periodically and thus, know when (and what) they need to update. The other mIRC's could then update the same file to indicate that they have read the new data. You could even use the information in the file to create a sync tab on the client's screen.
Also, I've never used DDE in mIRC, but maybe it would be suitable here to notify the other mIRC's that something has changed.
-genius_at_work
