You can have Starter 1, followed by Tandem 2, 3, 4, etc., or Starter 1, 2 followed by Tandem 3, 4, etc.
You cannot have Starter 1 followed by Tandem 2 followed by Starter 3. Once a Tandem enters the rotation, only other Tandems can follow.
In your case, if Tandem 2 is locked to start, you can set your next intended SP into Starter 1, and once the tandem pitches and Starter 1 is locked, then you can remove Tandem 2 and align your rotation correctly.