Why does my shower run out of hot water quickly?
Replacing the tank eliminates the most common suspects, although it is conceivably possible that you've just had terrible luck and there's a similar issue with the new tank as well. You can test the capacity of your water heater using this procedure, filling buckets with hot water and measuring the temperature to see how much you can draw down before it gets cold.

Assuming the tank capacity is fine, then the remaining possibilities are that the hot water can't get to you (which seems quite unlikely when there's no tempering valve), or that you are simply using it all up. If your shower heads have really high flow rates (like >5GPM), it is possible you could actually be using up all of your hot water in just 10 minutes. You can measure the flow rate of your shower head by using the bucket from the capacity test and timing how long it takes to fill it.