On Mon, 5 Jan 2009 11:17:11 -0800 (PST), wrote:
I have a 14 year old electric water heater (40 gal, 2 4500w
elements). The water temp begins to cool down significantly in the
middle of a second shower. There are only 2 people in the house.

Thermostats work - both come on and off normally depending on how I
set them and they properly apply voltage to the heating elements.
Both heating elements show 15 ohms and draw 17+ amps.

Anybody got an idea what the problem could be?

Your two showers are using more than 40 gallons of hot water.

Turn the temp up so you use less hot water, get a water conserving
shower head, or quit being so lethargic in the shower.