In order to link an additional developer account, go to either: 

On this page you have an overview of all your existing connections and you can also add a new one at any time by clicking on the Add Account button in the Add a new connection account section.

This will open a new pop up were you continue by choosing either Google Play or Apple App Store platform.

Here you'll be able to connect additional developer accounts. Simply click on "Add New Credentials" and enter the App Store Connect or Google Play Console login.
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Note: See how you can connect your App Store Connect or Google Play Console accounts.