Setting up a secondary mail server in Post.Office is very similar to the way you would set up 'sendmail' to do the same thing.
In sendmail, you would edit the relay-domains file to include the domains for whom you want act as as secondary mail server. These domains would not be included in the local-host-names.
So, in Post.Office, what you want to do is to accept the mail to selected domains, but not to try to deliver it locally. You will essentially be relaying this mail.
Make sure that in the "Restrict Relay Settings" you add the domains for which you want to act as a secondary:
If relay mail is restricted as specified above, use the following delivery options:
Allow delivery to
Check: Additional Domains
Then list all of the domain names and host names, for example:
You will also need to make sure to configure the proper MX records. The secondary mail server should have a lower priority (higher number) than the primary in each of the domains that will be hosted.