Hi Nancy,
When connect multiple CNAMEs/domains to one instance of marketo (i.e cname.domain1.com and cname.domain2.com) - the pages you create are always automatically linked to ALL the domains.
As an example - if you create a page called page1.html the same page will be available at:
cname.domain1.com/page1.html
cname.domain2.com/page1.html
You don't need to change any settings to achieve this just test it out in a browser.
Warm regards,
Nora