I have recently moved into a two story house with two bathrooms. Whenever two taps are opened we have found that the water pressure drops to almost non existant. (eg if a shower and the washing machine are going at the same time.)
Are you on a well. Do you have PRV in main line and it is not ajusted right, check it out and if you have PRV loosen nut on top and turn screw clockwise, Check and make sure main valve is on and valves on fixtures are on, if you still have problems call water co. and see if press. is low. You can buy a press. guage the you can screw on hose bib to check press at Box Store. Let me know.
What kind of water supply piping do you have? Many older homes have galvanized piping that can corrode on the inside over time causing a restriction in flow. There truely is no “simple fix” for the problem you’ve described. You need to have a professional out to evaluate the problem and provide a solution. Too many possibilities to diagnose over the computer.