Can't say the tandem helps any more for losing weight than riding a single bike.
But it has helped me kick start Gina back into riding shape at times. Meaning there have been times she has been off the bike for a few weeks or so then returning to her single bike she has struggled to keep up with the boys. So I put her on the tandem during the week and we ride. It's like my leg speed gets her moving again. She has to since the pedals are connected. It's like a kick start to get her legs whipping in motion again. Then she returns to her normal self.
As far as losing weight? Only when we ride it a lot. As far as me being the captain (front rider), it is a good workout when we climb hills. I don't think it helps me lose weight but I sure do think it helps me get stronger as I am the stronger climber and it is a good workout helping a slower climber up the hill. Don't get me wrong, she is working her butt off up the hill too or I would feel it and complain. SO yes, we both work hard going up a hill, no slackers!
One thing for sure though, riding the tandem is fun!
You can see here we are doing pretty good keeping up with some strong riders.
Video 1:30 in, On the return section of our ride, final 10 miles of our 40 mile ride, we are averaging 21 MPH.
Lots of fun!