Perhaps we have a misunderstanding.
The Manifold joins the two cylinders so, if it is open you have a 24 liter capacity. The manifold is below (on the tank side) of the pillar valves.
The pillar valves from this 24L space connecting the two DVs. if a DV fails you just shut its valve and continue breath on the other one. You are still breathing the whole 24L lump.
If you hear bubbling and you don't know what it is you shut the manifold. The system is now two 12L cylinders, each with a separate regulator. Now try and shut the pillar valves and try and stop the bubbling. Whatever happens you have half the remaining gas on one reg to go home on. If it turns out that a first stage or a hose has broken and shutting a pillar valve stopped the gas loss leave the pillar shut and open the manifold and you can breath all the remaining gas. If it was the pillar itself broken you won't stop the bubbling but you still have gas to breath and if you are diving thirds and you were quick on the manifold you will get out on your own gas.
Manifold pic