Are both accounts set up through BWC? It's pretty much effortless if one account is set up via BWC and the other via BES. You don't have to worry about the "reply to/sent from" address unless you want to switch and even then it's simply an option in the header of the email that you are drafting.
If you're stuck with BWC for both accounts, I've heard that Pocketday and one other app (bbreply?) will allow you to choose from several "reply to" addresses. I haven't used either myself so I can't really comment on the effectiveness of the apps.