Some other info I forgot:
Spamassassin (with the default setup) does not seem to run on the backup server when it receives an email to be relayed! It *does* run on the primary server once it receives the email from the backup server, so I don't see any need to change that.
I'm leaning towards a "push" implementation (using SSH and SCP like Morons above suggested) from the primary to the backup. At worst, this push could be run on a cron job. At best, it could be hooked into ISPConfig whenever there's any changes to the email system! Even in the second case, a cron job wouldn't be a bad idea incase the backup system was unavailable when the job was originally run.
Also, by the looks of it, this can be easily setup as a two-way street. Server2 act as a backup to domains on Server1, and Server1 can act as a backup to domains on Server2. It's a matter of keeping the relay_domains file updated with domains to act as backup to on the other server.