how do i redirect my second domain?

I have two domains, ie. a.com and b.com, both can visit. with SEO URL turned on.

Options +FollowSymLinks
RewriteEngine On
RewriteBase /

From Ultimate SEO URLs

RewriteRule ^(.)-p-(.).html$ index.php?main_page=product_info&products_id=$2&%{QUERY_STRING} [L]
....

now i just want my site showing b.com no mater a.com or b.com is entered...i only want my url is show b.com.

what should i do?