I have posted a question before about this shower/tub. I was able to solve the problem based on the info I got on this site, so I will try again.

In my house there is a shower/tub, and then a second shower (have 2 baths). The second shower has hot water until the cows come home, but the shower/tub will only have hot water for 10-15 minutes before the hot water runs out. The shower/tub has stronger water pressure than the freestanding shower... is there something I should do to the hot water heater to give me more hot water? We can't even fill our bathtub with hot water without it turning cold.

It is possible that the Cold water pressure is much higher than the Hot water (shower/tub) & when you open both the taps (Hot & Cold), because of the pressure difference, the cold water is also going into the Hot water tank through the hot water tap/pipe & hence cooling the Hot water tank. Is the cold water on Mains pressure ?

Yeah. We have city water so it's on a water main.