Not to put a damper on things here, but I think it is time for a reality check of sorts. Many of the configurations suggested do not really meet the redundancy requirements of solo diving.
Some of the solo divers posting are stating they use a normal rec setup with no redundant air supply other than the surface and are relying on an ESA to get them out of a sudeen OOA situation. It is always a good idea to limit your depth solo diving to no more than twice the depth you can free dive to, but not having a redunant air supply leave you vulnerable if you are ever prevented form leaving the bottom for any reason, even briefly, if that event coincides with an OOA situation.
Only slightly better are divers who are using H valves. H or Y valves provide no insurance against blown neck o-rings, burst discs, etc where the leak cannot be stopped or isolated. They also require prompt action by the diver to identify and shut down the offending regulator in the event of a free flow. That reuires a good deal of proficiency, training and ongoing practice - more than just seeing if you can reach your valves. Even with good skills and technique, you are only working with a single tank and far less volume, so any air loss in the time it takes to shut of the offending reg is far more critical than with doubles.
A set of independent doubles, or isolator manifolded doubles are far better options, each with different advantages and disavantages. Both offer more air, and you can never have too much air solo diving.
Independent doubles offer the advantage of requiring no action in the event one reg freeflows as the other tank is entirely separate and the logistics are much easier if you do short/shallow dives in addition to longer/deeper dives.
Isolator manifolded doubles offer the option of diving with the isolator closed, like independent doubles, but with the added option of turning off a free flowing reg and then opening the isolator to access all the air in both tanks. The isolator can also be left very slightly cracked open to allow the tanks to equalize to simplify air management yet minimize air loss from one of the tanks if a leak develops in the other tank/reg. Or the isolator can be left open and at worst you still have more time and air volume to play with compared to a single tank with an H valve in the event you have to shut something off.
Finally, some divers are relying on small pony bottles and this can often lead to a false sense of security. Simply put, regardless of math computations that state otherwise, 13 cu ft is not enough in the real world. If you are solo diving 19 cu ft should be the minimum and 30 cu ft is better for shallow diving and required for deep diving.
In the real world an immediate ascent is not always posisble, a slower ascent and longer saftey stop is almost always better and small ponies are often not as full as their owner thinks. Each reg check and each time the reg is pressurized air is used that in very short order reduces the air in a small pony.
Regardless of the pony size used, the pressure has to be checked before each and every dive and the diver has to ensure the pony tank/reg is on if it is back mounted.
Training and experience issues aside, if you do not have the proper equiment to solo dive, you should not be solo diving. Period.
