By default you are unable to run more than one MT4/MT5 platform from the same place at the same time.
If you wish to run two instances of MT4 or MT5 platform on your PC, you will need to install a second instance of the platform but in different directories on your PC. When you are running the installer, there is a setting button on the same page of the licence agreement. Click on the button and rename the destination folder and continue. It will then install another copy of MT4/MT5 on your computer.