Most of the tour ops will normally quote for a 14 days stay, but not always via the internet booking systems. Price is normally twice the weekly rate, less a deduction for the flight and transfer. Try Inghams, Neilsen or Thompson etc, dont know if Balkan do them for 14 days, or you could of course book via this site I guess.
We've done 14 days several times - including a consecutive week in Bansko and a week in Pamporovo. We just phoned Balkan and they sorted it all out for us. The only thing we had to do on the twin centre holiday was to arrange our own transfer between resorts but that was easy to sort.
Good idea pete, I love Bansko, but i expect I would find 14 consecutive days too long due to resort size, a second week in another bg resort would certainly make for an enjoyable fortnight, and all for the price of the average 1 week holiday in many alpine resorts.