I have tried tandem pitching and I have never succeeded in being able to have success with it. First off the two starters need to get to the 7th or 8th inning throwing 60 pitches or less each because there are only three rest days between starts. Secondly it takes 8 pitchers to fill out the starting tandem roster. Third part comes from the pen as you only have 5 available and one is probably the closer. The other four need to be set up as tandem also, two available and two at rest. Those four need to be long relievers also at least.
Here is why it always failed with me. For some reason one of the opening starters doesn't get out of the first inning or doesn't make it through the second, that hurts the pen. One of the starting tandems gets injured, puts a quick kink in things. An injury to the pen isn't helpful but maybe not as devastating. Long extra inning games seem to happen in bunches, quick end to the pen and no way to rest them except to bust the tandems.
I have tried it with 14 pitchers on staff and that hurts the offense in more ways than one. A backup catcher and only two spare players that need to play just about every position and hit well are hard to come by.