How's that?  My understanding is that back pay (which I'm assuming you're using to refer to either the shelving of shares, or the payment of previously shelved shares -- if you're referring to something else here, please clarify) simply goes on a share-by-share basis and has nothing to do with which account a share is under.  If I submit 1000 shares under Account A, and 500 of them are shelved, that's the same as submitting 500 shares (250 shelved) under Account B and 500 shares (250 shelved) under Account C.  The order in which the shares go into the system is the only thing that matters.  Assuming that hashpower is equally divided between Accounts B and C, and thus they are both submitting shares with approximately the same frequency, I don't see how one's overall payout would be affected by splitting up in this way.  (Aside from the payment queue, which of course is managed on an account-by-account basis.)