You can’t log in to multiple trading accounts in one MT4 at once.
In this case, please install two MT4 in different folders in your computer, and log in to your trading account in each of the MT4.
When you install more than one MT4, make sure to select a different folder other than the previous one. Otherwise, the new MT4 will overwrite the previous one and you will eventually have one MT4.
As another solution, you can also login to multiple accounts on one MT4 platform, and save the login details there.
This way, you can switch accounts instantly on one MT4 platform.