Each ride works a little bit differently, depending on the configuration of the queue area. Be sure to ask a CM when you get to each one. Basically, the first adult, plus any children who are big enough, get in the regular queue. As they enter, they are given a pass for the second adult. The second adult is told where to wait for the first. The first waits in line and rides the ride. He/she then finds the second adult, takes over child care duties and sends the second on their merry way, pass in hand. When the second adult (and any children who want to go again because they were tall enough for the first ride) gets to the queue area, the pass is surrendered and he/she enters the FP or single riders line (or whatever the rules are for that particular ride), rides the ride and has a great time.
We used this a bunch with our 2 DS last year. Our older DS was tall enough for Test Track so he rode with both of us. He was too small for the others so we took them both to a nearby playground or arcade and met up there. Mission:Space is particulalry well set up for this. They have a playground right inside next to the ride exit. My boys hardly even knew that the swap had taken place.
--H