This morning I measured the flow rate of my shower head with the flow rate set to full on. My shower head is indeed a 2.2 gallon per minute shower head. I just set the volume knog in the shower to the flow rate that feels comfortable. That comfortable flow rate is about 1 gpm,and about 1/3 of the water is from the cold valve.
I would not be surprised if I could run two simultaneous showers even with the current low 40 degree incoming water temperature. I will test that after I get my second shower finished up.
Obviously, some people do not like "low flow" shower heads. They require something more like the Commando 450!
The IPC commisioned a study about the flow rate required to take a shower. They found it hard to complete the study.