Planning to test exchange 2007 and 2003 simultaneously.

Hi,
I am having two servers. First one is Exchange 2003 front end server and other is back end one. I am planning to test exchange 2007 and 2003 simultaneously. E.g. Aminoo mailbox should work on both Exchange 2003 and 2007 simultaneously.
Can you give any tips on how I can accomplish this?
I need some guide on how to make this work. Any help will be much appreciated.
Thanks.
