Different domains link to one vB?

[TehForum]

I got two domains which I want to link to my main domain, so let's say the main domain is called 'URL 1', well I 'URL 2' and 'URL 3' which are not being used, and users to access 'URL 2' but they will be linked to everything 'URL 1' has.

I don't want no redirection stuff, if you still don't get what I mean then let's say there is a thread in 'URL 1' which is the main domain, well I want to view the same thread but under a different domain; 'URL 2' so it's 'URL 2.org/showthread.php?p=1748#post1748' which is the same URL link as 'URL 1', so it's 'URL 1.or/showthread.php?p=1748#post1748'