Davinci Tandems already makes an independent coasting system for bikes. http://www.teamspirit.net/dv2.html
On a 90* oop tandem you can still make both cranks point up. The worst is 180*oop, and then you can still get them both horizontal, which clears just fine on a turn.
If you don't have time to hit a lever to throw your hub in to neutral, how are you supposed to have time to throw your crank in to neutral?