two URL's in one

Different web hosting companies have different names for these features. I can tell you how to do it in Apache, but because you mention “add-on domains”, I assume you are using a traditional shared hosting company.

I believe that Parked Domains are what you are looking for, but I can’t be sure unless you tell me who your web host is. Of course, you can just try it for yourself. Or, better yet, ask your hosting company.

hostgator, I’m not sure just adding it as a parked domain will work. I think this will direct to the root domain, I want it to share with an add-on domain.

Hostgator documents the process here:
https://support.hostgator.com/articles/cpanel/how-to-park-a-domain-on-an-addon-domain