Blackberry receiving emails from BES and other Exchange server

I have a user that needs to do the following.  Receive emails from Exchange from Company A which is using an SBS Server.  And the blackberry needs to be on a BES from a Company B.  I understand a blackberry can't be setup on two BES's.  So I need to know how to configure the Exchange on Company A so they can get their email without.  It has to be setup so that Company B is pushing emails to the blackberry via BES and Company A is allow the blackberry to connect to the exchange another way to get the mail.