My second bathroom shower will have enough hot water for only one shower but if I go to the master bath there is still plenty on hot water. What's up?

You don't give enough information to answer. What size water heater? Gas or electric? How far is each bath from the water heater? Is there a flow resister on the master bath shower head and not on the other? How long a shower are we talking about?
Is there a recirculating pump and line installed? All these factor in as to the usage.
Experts! Feel free to jump in with anything I've missed. Cheers, Tom

Please answer Toms questions.

Are you saying you run out of hot water in the upstairs bath but still have hot water in the other bath, or that hot water last longer in the other bath than the upstairs bath?

If you run out upstairs, but still have hot water in the other bath, are you sure you don't have two separate water heaters?

Might be a defective temperature balancing device in the 'cold' shower.

Is it a single handled type shower?

If so start with that...

I had the exact problem at a customers house and found that manhattans thought was correct. Her moen poitemp cartridge needed to be replaced.