Overall, it's an evolving condition. To add to what the more seasoned teams have already commented above -- we're only 9500+ mi in 7 1/2 yrs -- is balance key when the stoker starts moving around. It only took my stoker/navigator/Admiral about a mile before she figured out how to move around w/o upsetting the balance & cg. You'll notice instantly if she starts twisting her body around when the bike is starting to lean in that direction. Most stokers whom I've met say that they master THAT point very quickly. If your stoker is going to turn around to turn on the rear light, she should inform you what she's doing so that your can compensate. She doesn't want to crash any more than I want to.

Cadence. Another potential sore spot. But you'll end up compromising. On singles, I tended to mash (75-80 rpm) & she spun (85-95). We settled on 85 (w/ a minimum of 80 rpm). And we eventually moved up to the 85-95 range. We use 90 as our cruise and shift by 95. My stoker will let me know if I'm in the wrong gear -- chain ring and/or cassette -- but the computers keep us honest. Dual computers are a must.