I think I've just about read all of this thread now
I'm intersted in this because I have 2 accounts on Betfair. I had the original one for ages for betting on the exchange and never played on Betfair poker. I set up a 2nd account just for poker (All accounts are created on the exchange site). Then after a particulary bad Saturday night
I found I had no money left in my "poker" account but wanted to play in the WSOP qualifier on the Sunday night. So I logged in with my other account and played the sat. Now as it happens I have never bothered to deposit on the original account since and only play the 2nd one, so in effect I have never had two accounts "active" at the same time on BF poker.
BUT I can find no mention of this in Betfair's T & C's (I checked because it didn't seem right to me at the time).