Looker Studio only supports having one set of credentials per connector. When you configure the Matomo Connector for Looker Studio with a specific instance, every data source you create under that Google account will be for that specific Matomo instance.
Later on if you find you need to change the instance, you can do so by revoking access for the connector. To do this, open one of your reports and click the Add Data button as if you were going to configure another connector. Search for Matomo, but do not select it.
Instead, click on the three vertical dots to open the context menu and select Revoke Access.
Once access is revoked, you will have to reauthorise the connector, which will allow you to use a different Matomo instance.