General derailleur advice for building up a road tandem:

1. Use a road triple up front, with the correct curvature for 52-53T.

2. Use an MTB derailleur in back, preferably with the needed cog capacity and a lot of chain wrap (long cage rear derailleur). Many people have forgotten about chain wrap, because the specifications are no longer to be found in the catalogs. For Shimano at least, I think that can be found at their website. I have found that shifting quality does not suffer when using an MTB RD on a teeny-tiny (11-21) road cassette.

Do not be surprised if after you have the shifting adjusted to perfection on the workstand in the garage, you find that with riders on board the indexing is now imperfect--the frame bowed with two-up riders.

When I tour in Colorado, I often put together a chainring and a cassette intended specifically for that use--if you are only going up and down, you don't need the middle. My experience on a single bike has been that the road FD will shift across 24-36-50 satisfactorily if you slide it down the seat tube a little, and that the road RD (Ultegra triple) will handle almost all of the chain wrap. With this adjustment to the front, and a 12-27 on the back, the steeper portions of Rocky Mountain National Park, Cottonwood Pass, and the Mount Evans race can be handled at a competitive rate. No other drivetrain changes were needed. Note that the 24 X 27 low gear gives less than one-to-one gearing on 700C wheels.