With enormous respect to DIR-Atlanta, I want to disagree. You can balance out most BC setups, using trim pockets or camband weights or tank neck weights. But the "donate the primary" concept is very central to DIR, and it is my belief that it is a safer way to plan air-sharing. It is an inexpensive alteration to put a 5' hose on your primary, and necklace your backup. You can then get used to managing your hoses like that, and work on your trim with the adjustment of weight on your existing BC until you can afford the backplate and wing setup.
I've had my bungied backup save my own bacon a couple of times; I'm a real believer in that setup. You CAN bungie your backup and use an octo length hose on the primary, but it routes under the arm and comes up to your mouth at an awkward angle, and often requires an elbow fitting to allow the reg to sit comfortably in your mouth. If you don't route the hose under your arm, then you have an ugly, large loop of hose which is just asking to get caught in anything you swim near.
