I tried the trick with a tertiary entry matching the primary, but it didn't reduce the spam at the secondary very much.
It would be useful to figure out why this is so. Did you use the same host name for both primary and tertiary? Or same resolved IP address? Does sendmail do any optimization like dropping candidate hosts found more than once in the MX list? Could it be that some ratware also makes this optimization?
I only have the one IP but I can create many hostnames in my domain to point to the same mail server and use that technique.
Yet another reason to switch to IPv6, so we'll have a glut of extra addresses to hide within.
