Mapping Multiple Domain Names – Two As Primary

Okay, I have a domain mapping questions that's making me run circles in my head. We have a WPMU network setup using subdomains. We use domain mapping plugin to map domain names to these sub domains.

We have one site in particular, where the client owns multiple domain names. So, we've entered all domain names into the domain mapping plugin page. So now we have say 4 domain names all mapping to one main domain name.

There is one domain name in particular that we what to make sure remains in tact in the address bar; without redirecting to the primary domain.

Ex. domainname.com – set as primary domain name

anotherdomainname.com – redirects to domainname.com

yetanotherdomainname.com – redirects to domainname.com

domainname.ca – remains domainname.ca in the address bar

*We don't want to disable primary domain check on our entire network. And we don't want to disable primary domain check for this one site. (screenshot attached)

We do want to make sure that one of the domain names being mapped does stay in tact in the address bar. Not sure if this can be done with a CNAME for this particular domain name, or if we can somehow accomplish this in the .htaccess file, or via functions.php theme file, etc……

Anyone have suggestions for how to accomplish this? Any help would be greatly appreciated!!!