I'm not exactly sure how the Da Vincci independent drive chainsets work but if they do indeed allow eiher rider to pedal independently in a different gear, it woul be interesting to hear how riders using this drive train relate to any of the above. It think it is a little more complicated than this since riding together when linked in a conventional drive train may give more feedback to each rider and help improve any discrepencies in pedalling, as well as potentially reduce mechanical losses, but it still would be interesting to know.
I'm a happy daVinci stoker, and I can at least say that the captain and stoker do not pedal independently and do not pedal in different gears. When both are pedaling you feel that resistance and pedal at the same cadence just the same as you would with a conventional tandem. What the daVinci will allow is for the stoker to take a break and coast for a bit while the captain is still pedaling. Likewise the captain can stop pedlling and coat while the stoker continues to pedal. When either of the two coast, particularly if it is a slight hill, the bike will lose some momentum since you don't have two people putting forth effort. The ICS (Independent Coasting System) is useful for standing, stretching, fumbling in the rack bag and getting water. Also useful for starting out while on hills or just waiting for the captain to get fully clipped in while the stoker pedals.