So you have tandem 1a/1b and tandem 2a/2b right now?
If that's the case you should be able to change your Tandem 1a guy out into a starter #1 when your Tandem 2a guy has the ball. So then you'd have starter 1, tandem 2a/2b. Then once starter 1 has the ball you can change your 2nd tandem into whatever you want (starter #2 only, starter #2 and starter #3, etc)