We tried it when we used to have a tandem years ago. It works very well for hills- if you are fairly evenly matched, the output torque is much more constant and you don't slow down nearly as much, this being the big drawback of tandems- they are MUCH harder on hills.

The big drawback is in starting off. The rear rider has to have one foot (usually the right) on the pedal at twenty past or twenty to- which means that the front pedaller has to put in all the initial power to get you moving and balanced. We worked it out after some practice, but we doing a right turn in heavy traffic was nervewracking.

There's also the problem of coordination. The stoker has to be well warned and quick on the uptake when starting off and stopping, otherwise they can easily get a smart rap across the shins as the pedals start or stop unexpectedly. With the pedals in line, it's much easier to follow the leader as you work in unison. I suppose people who are good at singing rounds together should be good at deviant tandeming.

So if you fancy a go at tandeming, and have a good powerful initial thrust (ooer missus), strong arms to keep the balance on starting, a pal with good anticipation, and a quiet road to practice on, I think it's worth having a go.