I had a tandem for many years and the pedals were offset initially. I had to change that for riding was much more difficult. The reason: Both riders were not "swinging in tune" which made steering exactly much more difficult.

With a tandem you realise more than with a normal bike that the steering is not done with the handlebars but with shifting the weight first and only then use the handlebars. Once with my brother behind me we were cycling on a cycle lane which had a pole in the middle coming up to us. I had to slow down to a standstill before the pole and then I turned to him asking: "Say, you thought I wanted to pass on the right of the pole?" He said yes and because of that i was unable, though having the handlebars at my command, to pass to the left of the pole as I thought was the better way.

I often stopped for hitchhikers (they usually didn't dare to say no thanks I wait for a car) and with someone not used to a tandem the pedals being not parallel was really bad.

Last argument: If it would be any good you'd have seen it at the Olympics or world championship tandem competitions.

Picture of the most successful tandem, Morelon/Trentin, offered as proof

Wolfgang