I honestly don't know.
What I have done myself, is pointed my incoming email servers at
www.mxguarddog.com for spam and virus filtering. Basically for a small text link on your website, you get industry standard spam and virus filtering for free.
From that point the email goes to the main mail server and if it passes the check it's legitimate the email is delivered. Can I suggest, looking at someone who offers free hosted dns. These people may be able to help get things set up the way you want to. It does though seem odd if you want emails from one domain to go to two mail servers.
I've not done it, and personally just using spam filtering can solve most problems these days.