Each seat has its own supply. Otherwise it's not a legal seat.(there are certification requirements to carry passengers)
If your mask doesn't deploy for you, the theory is sound that you could share a mask. Good luck getting it to happen. Then again, it would give you something else to think about and work on during the crisis so that would be good.

This is like worrying about an Ostrich attack when you're at the mall.

The number of incidents of uncontrolled decompression, globally, is negligible.

Agreed Martin.

I can't fly the plane. Unless someone told me where the autopilot thingy was.

I quit worrying about stuff I can't control.

I've been told by a couple pilots that Microsoft Flight Simulator is accurate enough that a little bit of time with it you could fly a plane if you had to.
I got to fly with a guy who owned his own single prop 4 seater and he explained things to me and let me take off and fly, landing is the hardest part, getting the angle and speed just right.

"Hmmm... how much of that time would be used just to realize that SOMETHING is happening, come to terms with the situation and decide that the right action is to go get an oxygen mask?"

About as much time as it takes to realize that slamming on the brakes is a better option than slamming the accelerator to the floor when you see that speeding semi running the red light at that intersection.

As Lono said, if an oxygen masks drops in your lap, put it on.

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"I think the maneuver is to roll the plane on its side and unload the wings into something like a sideways 0 g condition. This drops the plane quickly and doesn't build up airspeed like a dive would."

I think that's called a side-slip.

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"Pretty hypothetical, but would it be possible to share air, as one does in scuba emergencies, if you could not reach your own air supply?"

Do the words 'From my cold, dead hands!' mean anything to you?

Sue

Nit picky correction: The maneuvers are similar but not the same. Side-slip doesn't unload the wings.It's the unloading that gets the plane down in a hurry.

Sideslip is used to create drag by putting the fuselage a bit sideways in the airflow. (Or as I used it- to move the passenger's head out of the way so I could see the runway on final from the back seat of the training glider)

Reconsidering- there's not a clear demarcation between where one ends and the other begins. But a side-slip is generally considered a normal flight maneuver used routinely for landing. The other is a highly radical flavor of similar control inputs.
Kind of like the difference between a racing turn on a dirt track and a bootlegger's turn on the highway (or something like that).

Current safety standards for commercial airplanes mandate that no single failure of the pressurization system can result in a loss of cabin pressure.

Modern airplanes can fly at altitudes above 40,000 feet. Certification to fly at these altitudes requires that no loss of pressure would result in the passengers being exposed to cabin altitudes above 40,000 feet for more than 1 minute. Similarly there is a long standing requirement that passengers not be exposed to cabin altitudes above 25,000 feet for more than two minutes.

This means that emergency descents must rapidly exceed 10,000 feet per minute (about 113 mph vertically). Assuming the pilots can maneuver aggressively enough to meet the regulatory requirements you would most likely pass out within 30 to 60 seconds and assuming no other injuries, you would regain consciousness after passing below 10,000 feet, say in 2 to 3 minutes. Not sure if this is enough time to cause significant damage other than from the rapid depressurization.

Even with 100% oxygen there is not enough pressure/density above 30,000 feet for most people to sustain consciousness. Pilot oxygen masks are sealed to maintain pressure. Passenger oxygen masks don't seal enough to maintain pressure but having a higher concentration of oxygen at even much lower pressures can help a lot.

-Eric