Advantages of the front drive (long chain) for the tandem setup include:
- more even bearing loading on the BB. Anybody who has destroyed rear BB's on modern tandems (with strong teams) will know about this. I had a Phil BB where the spindle kept working itself loose... because one side pulls forward, the other backward.
- Feedback when shifting. Important in pre-click shift days. With a rear drive, the captain doesn't get feedback and doesn't know whether the gear is engaged securely.
- Chainline and rod-operated derailleurs were mentioned already.
For Cyclo derailleurs, there were stronger chains available. We used one on our Paris-Brest-Paris tandem. No problems.
In France, cheaper tandems always used the "modern" setup with rear drive (see the 1930s Longoni in our book "The Golden Age of Handbuilt Bicycles"). The rear drive must have been seen as a major improvement to warrant the extra weight of the very long chain in those weight-conscious days.
For single-chainring drives, you can use a same-side drivetrain (two chainrings on the rear), as used on many track tandems in the old days. -- Jan Heine, Seattle Editor/Publisher Vintage Bicycle Quarterly c/o Il Vecchio Bicycles 140 Lakeside Ave, Ste. C Seattle WA 98122 http://www.vintagebicyclepress.com