Re: waterheater elements
First of all only one element at a time is every on in a properly functioning WH.
You would need to check the thermostatic control to see if it is rated at at least 19 amps or 4500 watts or more. Although it will heat the water faster it won't add significantly to the amount of time it takes to run out of hot water.
If that is a 50 gal tank you should have more than enough hot water for a 10 min shower and most showers are much less than that.
You could have other problems. One, the tank my be installed backward, that is the cold is on the hot side and the hot on the cold side. Another problem that can cause this is a broken of missing dip tube in the water heater.
Be sure you live your life, because you are a long time dead.-Scottish Proverb