Hi Rick,
The "how to" of this question would be best answered by your hosting support team. Not all hosting accounts are created equal.
For example, I have two additional domain names on my main account at homebusiness-websites.com and one sub-domain (set up as its own website) and the SSI works across all of them and is only declared once in htaccess file in the root folder.
Best Regards,
Steve MacLellan