The only way I know about is by using a jailbroken iphone and requires some terminal skills.

First the setup the first Exchange account.

Then ssh to your phone to edit the file "/private/var/mobile/Library/Preferences/com.apple.accountsettings.plist".

In the file you will see your current Exchange account definition, just duplicate the entry in the file (copy/paste). In the second entry (which you just created), edit/change the value of 'Identifier' and 'ASAccountEmailAddress'.

Now if you go to the mail settings of the iphone you should see the account you just created. You should be able to set the new server, password, etc...

There is some questions about battery life with this method. If you try this, Please let us know how it goes.