You can have multiple trading accounts, but you can only be actively logged into one account on MT5 at a time. To access all your trading accounts simultaneously, you'll need to install multiple instances of MT5. Follow the steps below to achieve this.
Run the installer and click on "Settings".
In the Installation Folder, add a dash ( - ) followed by any identifier you prefer. For instance, here we're using "-2" for the second MT5 installation:
Click Next. That's it!