Domain Forwarding with MultiSite

I have 2 domains that I'd like to point to one subdomain, akin to robert.com and bob.com both pointing to bob.mysite.com. I have bob.mysite.com set up with domain mapping to bob.com, but I'm unsure what to do with robert.com. Currently both robert.com and bob.com are parked domains on the hosting for mysite.com. Is this a cPanel issue or .htaccess or something else?