, it is possible to have your handheld on two distinct BES servers.
That is what the service books can do for you. By default, your service book is set to DESKTOP. Simply change the service book for atleast one BES (using the desktop manager redirector settings) If you are doing this for an end user, use very descriptive service books like (companyX and companyZ) so that the user will easily be able to compose new messages for a specific BES.
Rebuild the encryption key on each server. (Probably try this from the desktop manager using the USB cable)
I have successfuly allowed my handheld to have email capabilities on two BlackBerry servers as well as the webclient.
I have not had success in getting the rest of the PIM service books to allow me to have two address books and two calendars etc so I have a primary BES that my handheld is on. It has all PIM functions enabled. On my secondary BES profiles, I disable all PIM sync except email reconcilliation from the BES manager on the secondary BlackBerry server. I have also disabled MDS on the second BES since there isnt a way to specify how to use a second MDS service book.
Also, I generally avoid doing Enterprise Activations after I get this configuration setup. It doesnt always work as expected.
Check out the Free BES Performance Pack