My understanding is that the main risk freedivers face is not being out of air, but passing out on their way to the surface (shallow water blackout).
I'm not sure having a spare air would help against that: you're not necessarily aware you're about to loose consciousness. Actually, using a spare air or equivalent while freediving comes with its own risks (barotrauma).
There are at least prototypes of freediving safety equipment designed to address the problem: The Freedivers Recovery Vest. A spotter/buddy would work as well.
