Let’s say you have a 500 IP pitcher and two 200 IP pitchers for your rotation.
You could have a Starter 1, Starter 2A, and Starter 2B which would like like:
Game 1: Starter 1
Game 2: Starter 2A
Game 3: Starter 1
Game 4: Starter 2B
or if you use the tandem role which would look like:
Game 1: Starter 1
Game 2: Tandem 2A starts, and Tandem 2B comes in after him
Game 3: Starter 1
Game 4: Tandem 2A starts, Tandem 2B relieves him.