The UK military divers have used inverted twins with a manifold to a single 1st stage. This isn't the standard manifold with a central isolating knob. Its basically two cylinders with din valves and a two into one manifold. The 1st stage is in the centre.
You have one cylinder open and one closed. No SPG necessary. If you start with 2X200 bar 10 litre cylinders You breath a cylinder empty, crack open the reserve cylinder and equalise the pressure and close the reserve, you have a 100bar in each. Breath it down empty, crack open the reserve and you have 50 bar in each cylinder and its time to go.
You have one cylinder open and one closed. No SPG necessary. If you start with 2X200 bar 10 litre cylinders You breath a cylinder empty, crack open the reserve cylinder and equalise the pressure and close the reserve, you have a 100bar in each. Breath it down empty, crack open the reserve and you have 50 bar in each cylinder and its time to go.