I made my own...
I mean I addapted a cheap tandem by putting "freewheels" on both cranks
The ' daVinci tandem ' is way out of my price range.

>>Has 3 bottom brackets instead of 2.<<
Can be but dosen't have to be [our one is 2]

My lady stoker loves it... and now so do I
See Thread "New One" for pic

You can change the "cadences between riders" if you want [if one rider is a lot faster spiner then the other] by having less teeth on one crank [I've done this as my lady is somewhat disabled and needs to spin slower]

There are lots of ways to go... just keep thinking outside the square