I have a bit of an alternative to Nemrod's answer. It is one I have contemplated for myself, but I already have too many tanks and combinations. The original Aqualung concept from Cousteau contained, within the USA patent (#2,485,039, October 18, 1949), the idea of a triple tank setup, with the third tank used as a reserve, just an Nemrod said above. But under the Cousteau system, the third tank was mounted upside-down, with the valve on the bottom. If you look at the photos of the Cousteau triple tank setups, you will see this valve at the bottom.
That gives you an option for your diver. You can purchase the triple tank setup with a standard double tank valve (two posts for regulators would be better for balance, and give you a center position). Purchase two sets of double tank bands for the system, and it would be good to purchase a valve protector for the upside-down cylinder. Simply mount them in this fashon, with the double tank bands offset for the third tank (either above or below the other bands).
This triple tank system could then be set up on a backplate, with a BC if you want, or on a BC single-tank hoopup. That would take care of the BC connection.
Now, the tricky part. Since there is no connection (yet) between the doubles and the single tank, you could connect instead the regulators. This would require a second first stage on the reserve cylinder, and that would then hook into the primary first stage via a LP hose. This would require a female end on both sides of the hose. But that could be accomplished by placing a AMF Voit Swimaster MR-12 Seat - 2nd Stage Poppet (Item #28, Part Number B20210A) between two regular LP hoses (part #25 on the Titan); this particular seat is reversible, and both sides will connect the male end of a LP hose.
In this way, you would have a single scuba unit with two first stages, one a primary for breathing off and one a "feeder" first stage which would feed air from the reserve cylinder into the primary first stage. Both regulators could have available LP and HP ports for monitoring primary rig and reserve cylinder pressures, if desired. Here is the diagram for the AMF Titan regulator, which contained this same part:
The way the Cousteau system worked, as described in their patent:
The third cyclinder C serves as a reserve; the gas contained init being available for the diver when he feels that the supply of gas contained in the two cylinders A and B is close to being exhausted. The capacity of this third cylinder is sufficiently large to supply sufficient air to the diver to enable him to rise to the surface sufficiently slowly. The diver can thus dispense with any instruments for measuring the pressure in the two cylinders A andB. The air is withdrown from the reserve cylinders C through a pipe O. Pipes N and O are providid with (valves) K and L respectively, which operated by the diver. While two cylinders A and B have been shown for the normal supply of air to the diver, only one could be utilized instead...
This would not act quite the same as the Cousteau system, as they transferred the HP air from the reserve cylinder into the other two cylinders, and this created quite a sound that the team divers could all hear, so they knew when a diver went onto the reserve cylinder. In this method, the pressurization would already be there in the first stages. The reserve cylinder would be initially turned on, to check for leaks and pressurize the system, then shut off to function as the reserver cylinder (but the regulator would remain pressurized, since the air would go to that first stage from the other regulator). When breathhing resistance was felt, then the third cylinder valve could be turned on. If the valve you use between the twin cylinders is a "J" reserve valve, with a 500 psi spring, it could also retain 500 psi in the outside cylinder on the other side, as an additional reserve (using a pull-rod, of course), and it would need to be reverse-mounted as described here:
http://vintagescuba.proboards2.com/index.cgi?board=instruction&action=display&thread=1160451692
These types of cylinders are available in Al or steel, and the one I was looking at was the OMS Diving cylinders. They had a twin 45 available with double tank bands, but they are not currently advertising the double tank bands. Whatever type of cylinder you get, manufacturing double bands, or even a triple band system, would not be difficult for a machine shop. Here is OMS's specifications on their cylinders:
http://www.omsdive.com/cyl_spec.html
Now, saying all of this, you would have a very unique system, one which the diver would need to train on and use in pool and then in open water on training dives to become used to its features. Without further instruction, this could be a system which someone not familiar with could abuse, misuse, and get into trouble with. So this is my disclaimer, that you must build and use this at your own risk.
SeaRat