You and I are cross posting between here and VAF. I assume if GeorgeC got a 9% increase flying solo, then 2 users would get 4-5% apiece, 3 users would get ~3% ea, and 4 users would see a 2% improvement.
I have a G5 and LOVE it. I use it anytime over 10K' MSL, 5K at night. Why not, it's an unlimited supply, right? The owner of a company called InogenAviator (former Inogen distributor) sells a dual cannula setup with dual flowmeters that he claims is good for two to 14K'. He also sells a nice 28V capable power supply for the Inogen units. The challenge with multiple people on one system is the timing. Oxygen concentrators don't deliver a continuous flow like a tank. They sense when you start to inhale and deliver a 'bollus' of oxygen-rich air as you continue to inhale. It is like a puff of air in your nostrils. Between breaths, the concentrator is regenerating the next bollus. With two people, someone is going to get shortchanged with each breath. I don't think my system on max can keep up with double the rate of breaths taken. There is some capacitive effect of the oxygen-rich air in the cannula hose, but the machine won't be delivering a perfect bollus for each breath. I can easily raise my pulse ox by 10 percentage points breathing solo. It's more like 4 points with two people breathing on one system.