The shower valves are a great suggestion but will not help if the pipes are of galvanized steel.
Galvanized steel pipes are notorious for decreased diameter because of corrossion build up over the years.
When a 3/4 water service pipe is decreased by half the volume of water the problems you are experiencing will happen. When this volume is reduced again by a second shower you will receive “shower shock” or a decrease in temperature to your shower. Do you experience this “shower shock” when a toilet is flushed?
A good test would be to go to the hardware store and buy a pressure gauge that you can install on the house bibb at the front of your house. Open the hose bibb to view water pressure from the street.
Now go in and run one shower returning to the pressure gauge to see if it has dropped. Then go in and run the second shower leaving the first shower running, return to pressure gauge to see if it has dropped some more.
If as I suspect the pressure gauge drops each time the showers run, the gauge may have dropped by as much as 30 or 40#. If this is the case at your home shower valve replacement will not 100% take care of your concerns.
Sometimes it is the service line from the meter to the house that is bad, other times it involves a re pipe of the entire house.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com