What I've read is that if the couple is close to the same size (define 'close'...), it is more efficient to have the stronger one in back. If there is a substantial difference, it is a) harder to design a conventional tandem for the taller one in back, (think of the extreme case where you really want the stoker's stem mounted higher than the captain's saddle) b) harder for the smaller, lighter person to control the bike, and hold it while not moving, and c) less efficient aerodynamically.

In the extreme case, a) is undeniable. b) can be worked around - stoker may need to put a foot down when stopped, etc. Blind stoker/sighted wife/captain couples have proven it can be done. I don't know whether c) has actually been proven in definitive wind tunnel tests, or is at least part folklore.

Of course there remain the unconventional design cases, such as recumbents and semi-recumbents like the Bilenky ViewPoint.