Yes - since DiRT is restoring entire database tables if the failover keys have already been created it'll blow them away and completely hose your failover config and make a hash of things - you want to do the backups on the failover server and do the restores before doing the failover config - this is the cleanest way to go about it.

COBRAS does not do table restores, it's doing object creation one at a time - this is how it can "merge" objects from multiple systems onto one box and such. It's slower, doesn't port as much data and involves much more input from the administrator to work properly but is considerably more flexible. If you're primarily concerned with users (with greetings, passwords, voice names and messages of course) then COBRAS will work fine for you. This assumes you're running at least version 4.0(5) with Exchange or later (there are no restores onto Domino based systems).