By default you are unable to run more than one MT5 platform from the same place at the same time.
If you wish to run two instances of 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 MT5 on your computer.