Yes, it is.
You have to configure the domains on the remote relay and do “relay all” for every domain. Unless you want to go through the pain of manually re-creating specific mailboxes, anyway.
Then you have to configure a forwarding host on the main mail server.
Set the spam filter on the main server to disabled for the forwarding host (just the forwarding host). Make sure the backup MX server has its spam filter on or you’ll get spam.
And make sure the backup mx server is in your SPF records!