I don't know if you have looked at these yet?
http://cgi.ebay.com/RECUMBENT-TRIKE-...QQcmdZViewItem




Each rider has their own drive train / shifters so it would meet your requirements as far as each choosing their own cadence. Price wise, the last auction went for around $2,000
A couple of negatives / questions I see with it are:

(1) Are those block pedals and will straps / clips not be an option?
(2) Is having the single wheel in front safe for a trike design? I am thinking of how they stopped making 3 wheeled ATV's.
(3) If used on a lot of urban bike trails, how well would the double
wide be accepted by other on the trail? I know I often have trouble
getting groups of people to provide room for me as a single wide
to get by.

My only experience with Trikes is having a few pass me by as if I was standing still. I would be interested in what experienced trike riders would see as pluses and minuses with this side by side tandem.