Originally Posted by
L134
...That and, I think, even on a single one wants to let up on the pedal stroke when making the shift to reduce tension in the chain making it easier for it to release from the middle chain ring. This is a timing thing so the captain and stoker need to be in sync which makes the timing more difficult on a tandem....
+1. Based on my tandem experience, this is more critical that the cable issue, though both are factors.
Regarding the ability to handle a triple, my tandem was a 1985 Santana Elan, equipped with a Compe-V front derailleur. This is from the same era as the SL, so the latter likely has the same travel and can handle the lateral throw, provided the spindle length doesn't skew the chainline too far from the nominal cage position.
However, the capacity of the SL is stated as 15T. You are asking it to handle 20T, so you'll have to be careful with crosschaining and will not have as many useful combinations as with a wide range front derailleur.
Also, as noted, the SL cage is aluminum. Besides the higher wear rate, aluminum is not as stiff as steel. To compensate you have to make it thicker but this mitigates the weight savings. The cage may not be stiff enough to shift under some of the heavy load situations experienced on tandems. This is where the previously mentioned co-ordination between the captain and stoker can be critical.
The bottom line is that you can likely make it work but the concessions in gearing and performance may not be acceptable.