I actually won a WS one season using four tandems. But I was promptly bounced from the divisional series the next season, using the same method. So take all of this with a grain of salt.
I had a great group of batters, and they all stayed relatively healthy, so I only kept maybe 11 or 12 position guys. I then had a number of pitchers with staminas in the 50-70 range, and a decent group over for the bullpen. I put the 8 guys with the best staminas (and general ability) as my starting tandems, then I had another two LR guys, plus two setup guys and closer. My LR guys were pretty close to being good enough to be my last tandem (or was my last tandem bad enough that they could have been LR pitchers?), so if I was in a pinch, I could swap guys around from relief to starter.
The WS was not a product of my pitching, by any stretch of the imagination. All of my pitchers were average, or even below-average, but running the tandem system helped me spread out the crappy pitching so that everyone got a fair number of innings. It worked one season, it didn't work the next season, and now I'm back to running a regular rotation.