By default you are unable to run more than one MT4 platform from the same place at the same time.

If you wish to run two instances of MT4 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 on your computer.

Please note that MT4 trading platform is a RAM consuming software. Thus, running several platforms at the same time can slow the operation speed.