Well now I have the answer, I thought I would take the time to come back and share.
The answer is pretty easy really. Once you have the MX records set up, all you have to do is set the 'DS' option is sendmail.cf on the secondary server to point to the primary.
So in the example above, I would edit sendmail.cf on ben.test.com to say:
This tells sendmail to use bill as a smart host. And if you don't want sendmail to look the address up in DNS (eg. the address is already defined in your hosts file) then you would use:
You can also replace the FQDN with an IP address.
One last thing, you also have to add your domain (in this case test.com) to your access file - otherwise Sendmail will think you are trying to relay through it.
To do this, add:
to your access file. Don't forget to then turn the file into access.db by using:
makemap hash access.db < access
Then reboot and that should be all there is to it!