connect one handheld to more than one BES
Policy from the first BES will override the second BES, so choose the 'better' one (read: least restrictive) to activate on first. If you've already made the mistake by registering on a restrictive policied BES, then do a security wipe on the handheld (search for it on this forum) and then register on the other BES - then follow the steps to register on the second BES.
If policy agreements/restrictions are in place on either BES, then the administrator will notice (if they look) that policy has not been applied correctly to the handheld (for the latter BES). This configuration is not supported by ANYONE so it may break with no avenue for support. Also, it can be viewed as a security risk and/or violation, so be cautious in doing it.