I'm trying to replicate the Fabco Westcoaster tandem suspension that P.I.E. used. Why it was off centered, I do not know. Here is a few photos.
IMG_3173 by Brian Smith, on Flickr
Here you'll notice the center pivot is behind the rear drive tire.
IMG_1803 by Brian Smith, on Flickr
IMG_1791 by Brian Smith, on Flickr