What would cause a loss of hot water and pressure in one bathroom when you have water running in both bathrooms?

THE THING THAT CAUSES YOU TO LOSE PRESSURE AND HOT WATER IS BOTH BATHROOMS ARE CONNECTED TO THE SAME LINES. SO WHEN YOU RUN SOMETHING, THEN RUN SOMETHING ON THE OTHER SIDE, YOU LOOSE POWER THAT YOU WOULD NORMALY HAVE IF ONLY ONE BATHROOM IS ON. THERE IS NO QUICK FIX, TO FIX YOU MUST REPLACE ONE OF THOSE LINES WITH NEW ONES PROBABLY FROM THE BASEMENT. ITS ALOT OF WORK, AND COSTLY. JOE HOJAS