There are MIDP mail clients out there, which would totally separate the 2 email accounts, what you have to realize is that you will only get push email on the blackberry mail, not the 3rd party mail client.
You can somewhat separate the BES from the BWC in that you can specify a different return address for the BWC or BES account so that
goes through BES.
. You set the BWC reply from address to
and nobody will be none the wiser, it will all work fine.
On the blackberry, you can specify the Send Using option to reply and/or send email from whichever account you wish. You can also see individual mail folders for each account, the BES account and the BWC account.
I really think this is the way to go... I would rather have a glance at all my email and then be able to look at the individual accounts later.