The reason why this is important is #1. because if you are too far forward you limit the pedal stroke and will be less efficient. #2. You can't use the saddle as the anchor point to push from.

Picture the right crank arm as a clock face. If you are too far forward your stroke will start as late as 3 o'clock. The farther back you are the sooner you can start the stroke which is more efficient. Mine starts at 1:30.

Also you can slide back on the saddle, wedging your butt there and use the saddle as an anchor point (like the seat back on the leg press at the gym) to push forward on the pedal by gettting behind the pedal with your heel down.

It doesn't matter if it is a single or tandem.

You want more power you got it. If you are bobbing your head and upper body (compensating because your riding style leaves you without and anchor to push from) your setup/fit/style needs to be changed.

The proper way to set this up is to plumbbob a string from the bone just below your knee down to the pedal with a level crankarm. The string should be just aft of the pedal axle. Careful, you can go too far, and hyperextend your knee. When you slide the saddle backward you will have to adjust height down. Because of the angle of the seat tube. If you are running anything more than 73.5 degree seat tube you are robbing yourself of this setback advantage. (all those small frames) except for Orbea, Look, and some Lemond. Check the mfg spec before you buy.

Once your get the seat right the stem is next, to adjust your reach.

Womens specific designs suck, because when they shorten the top tube to be womens specific they stand the seat tube up. Where is all of a woman's power? In her legs, the lack of setback prevents her from getting all the power she can. You see a lot of women bobbing up and down because of this bad setup. Anybody bobbing up and down should be told unless you race against them, or are the club big mouth smartass.