I have a Thorn Me'n'u2 triplet (three seater tandem), and was having a problem where whenever I tried to accelerate suddenly (either solo or with stoker(s)), the front chain seemed to fall off (happened at least once every time we went out).
I tightened it up (removed a link and turned the offset bottom bracket) so it was then pretty tight (taught top and bottom, only moves a bit when squeezing the middle), and that problem stopped happening - so far so good.
Later on tour we saw someone with a similar tandem but their front chain seemed very saggy - they were stopped, and it was drooping very noticeably both on top and bottom. I didn't have a chance to talk to them, but presumably they weren't having the same troubles. Does this suggest something else is/was wrong on mine, and that it should be ok with a much looser chain?