I will try to help you then Mark can clear up what I mess up.

On any two-wheeled single-track vehicle (bike or motorcyle):

For any given head angle (say, 73 degrees), the more rake in the fork (the higher the offset measurement) the less trail the bike will have. Bikes with less trail will initiate turns quicker and with less resistance. Longer trail (less fork rake) will result in a bike that is slower to turn. Think Harley (lots of trail, wants to go strait) or Kawasaki Ninja (less trail, wants to turn).

Some folks rave about the Co-Motion 'racy' handling which in fact has more trail than other tandems which would result in slower steering. I have never experienced this first-hand. But, I would guess that on high-speed descents, the slower steering imparts a greater sense of control and stability to the pilot and thus the pilot pushes the speed of the descent.

Tandems are very stable at speed anyway because of the long wheelbase, so I can't quantify how much difference it would make.