I tried using different domains for joomfish in the joomsef config, e.g.
dutch <domainname>.nl (main site)
english <english-domainname>.com

Everything works fine except when using the .com domain you're not able to add the product to the cart (ajax).
I think the ajax prevent accessing from different domain. Is there a solution for this?

(Now I am using domain forwarding with frame to tackle this problem)

Well, it should not be a problem if the AJAX would always be loaded from the same domain the user is currenly on. So just make sure the call to AJAX uses variable and not hard-coded domain name.