Accessing WordPress Blog through another URL

Hi,
I have setup seven WordPress installations on a server that uses IIS6. Each installation has its own database, and resides in a sub directory on a website on the server (ex: https://wordpresshost.com/blog1).

I have another web server that also uses IIS and has 7 websites (ex: https://www.maindomain1.com), and each blog on the first server (wordpresshost.com) corresponds to a website on the second server (maindomain.com).

I need to be able to access the blogs by using a URL from the main domain server. for example (www.maindomain1.com/blog) will need to lad to the blog on (wordpresshost.com/blog1) BUT the URL in the browser need to stay as (www.maindomain1.com/blog).

301 redirect didnt work because it changes the URL, and using Frame and iFrame is not accepted in this case as we need the search engines to perceive the blogs on (wordpresshost.com/blog1) as part of (www.maindomain1.com) for example.

I tried changing the (home URL) in wp-config.php and placing a copy of index.php in (www.maindomain1.com/blog), but the problem is I am not sure how to instruct the index.php to get word-press files it needs from the other server.

I am new to WordPress and any help would be appreciated.

Thanks in advance