You should create two client IDs, one with the release certificate fingerprint and another with the debug certificate fingerprint. Make sure to use the same package name for both. This allows Google Play games services to recognize calls from your linked APKs that are signed with either certificate. For more information about certificate signing for Android, see Signing Your Application.
You can add more than one Android app to the same game entry in Google Play Console. However, this should only be done if there are different versions of thesamegame (for example, the free version and the paid version). In this case, always list the paid version before the free version (or the full version before the demo/trial version). Do not link different games to the same project as this may cause incorrect behavior. Instead, create a separate game entry in the Google Play Console for each game that you publish.