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.
If your broker offers a program “MT4 Multi-trader” where you can login to several accounts and trade together, then you may follow the instruction of your broker.