If you're going the Shiftmate route already, consider going with a SRAM MTB rear derailleur, X7, X9 or X0 (subject to your budget or what you find on sale).

This is the information on the Shiftmate Straight:http://jtekengineering.com/shiftmate_straight.htm

I recently fitted a SRAM X.0 rear derailleur to our road tandem mated to our Campy Ergo 10 shifters via a Shiftmate Straight 5S + Shimano 11X32t 10 speed cassette and the shifting is very crisp and smooth, an improvement over our previous configurations using Campy RDs with and without a conventional, right-angle Shiftmate. More info here, with follow-on anecdotals in the 2nd two updates:

http://tandemgeek.wordpress.com/2011...mpy-ergo-sram/
http://tandemgeek.wordpress.com/2011...-5s-follow-up/
http://tandemgeek.wordpress.com/2011...2nd-follow-up/