You can't. Facebook and most other social network applications can only work with one domain. If you are using multiple domains, you would need a separate Facebook Application, Twitter Application and other social networks apps for each domain. JFBConnect currently only supports 1 app ID and Secret key for each social network.
Unless you have 2 different Joomla sites, one for each domain, you likely won't be able to do what you're looking for.
In general, we always recommend having one domain redirect to the other. It's much better for SEO, social network sharing (all your Likes are consolidated on one domain/URL), and easier for your users. That's a general statement though as all sites are different.
I hope that helps answer your question,
Alex
multiple domains for site
