I am using the following Shimano components on our tandem:
6703 Brifters - Ultegra
7803 FD - Dura Ace
7800 RD - Dura Ace
11-28 Cassette - Ultegra
New 2012 Ultegra Tandem Crankset 52-42-30
IN MY OPINION the shifting is outstanding. I am totally pleased with the way it shifts, front and rear.
My road bike has 6600 Brifters and the throw on the RH lever is longer, I much prefer the 6700 series.
Wayne