I don’t think that the blackberry supports dual BES servers. I’m pretty sure its going to be one or the other.
The blackberry cannot connect to two BES servers. It is one or the other. However, you could forward a copy of your email from one exchange server to the other (i.e., the one to which your blackberry is connected). It would of course not reconcile with the initial server, but at least you would have it in your blackberry.