Domain aliases

Hi guys, been using boost on a large site and it's really helping with the speed , so many thanks .

One thing I need some help on. We have a domain alias for the website. The main site is at a .co.uk address and that is what boost is setup for.

We also have a .com. Pages accessed via .co.uk are being servered by boost. They are not with the .com .

I'm not great with rewrite rules , i'm assuming i need to add something extra to my .htaccess to get the .com pages being served by boost?

Help appreciated,Many thanks.