Is it possible to set the default file that is displayed as a URL

I know that you can do
DirectoryIndex filename.html

But how can you make that a url?

I have a htaccess code which for example displays a.com/index.php?option=com_content&view=article&id=41&Itemid=22 at b.com/index.php?option=com_content&view=article&id=41&Itemid=22. This is without using addons domains etc. Now if i can set the URL of the default file that is displayed as b.com/index.php?option=com_content&view=article&id=41&Itemid=22, b.com would display a.com/index.php?option=com_content&view=article&id=41&Itemid=22!

Then http://www.rockettheme.com/option,com_smf/Itemid,190/topic,39338.0/ would be solved, as b.com would display a page from a.com by default, and with my code that forwards requests to another site while keeping the original site, the whole idea would work.

So it just boils down to somehow setting the default file displayed as a URL!