If you change from singles to doubles a lot you should consider a BC that is easily configurable to your needs. The above mentioned Transpak II from Dive Rite would be my first choice, too. OMS makes a similar system. Due to their modular nature you can use singles or doubles, add backplates (to the vest, rather than use it with a harness), add weight pouches or pockets as needed. As 12 ltr. tanks are pretty large, I'd use a backplate, as it is more stable. The backplate slides between the vest and the pad, making it more confortable than just a metal plate by itself. Although a personal preference, I wouldn't use twins with jacket style bcs, twins are massive and balance better with wings. If you use a backplate, the tanks are bolted to it with two screws. From My experience, Dive Rite has fantastic service.