I've used tandems frequently and have never found it to make pitchers perform better than throwing the same innings in another role. Records are heavily skewed obviously — TB will only take a loss if he blows a lead during his 3-4 innings, and TA will have extremely few wins but more losses than if he were a regular SP. If your Tandems are throwing far fewer pitches than they are capable of, common sense says their performance will be somewhat better (i.e., someone who threw 7 IP/G in RL who pitches 3-4 IP as a tandem will have about 88-89% in-game fatigue when lifted, where he might be at 75% after 6 IP).