Yes, you can do this at those rides, generally any ride with a height restriction. The specific policy varies depending on the ride, but when you get to the CM attendant, tell him or her you'll be switching off. Sometimes, you go right to the loading area of the ride and wait for your other half, but most of the time, you'll get a pass which allows the rest of your party to return through the Fastpass line (but it's not a Fastpass).
We used it a lot on our last trip because DD was too young to go on any of the thrill rides, and we'll be using it again this time. It's a great deal. Plus, here's a cool thing, too: generally, the child swap gets you 2 or sometimes 3 return passes, so if you have a few people in your party, they'll get to ride twice in a row. (So my DS got to ride Space Mountain twice last time, first with his dad and then with me!).