Passengers on racks suck. They can';t help wobbling around trying to compensate for something. I've had the best results when they are nearly as drunk as I am, floppy enough not to react.
If they have slim hips, you might be better jamming them into a basket on the back, they can't wobble around as much. I did that a while ago with a 140lb dude on the back and another one about the same weight on the handlebars, but they had strict instructions not to wobble around. It was for about 3/4 of a mile, but mostly downhill. The dude on the bars probably acted as a good counterweight for the guy in the basket. This was on my tourer, so I was pretty confident it would handle 280lbs of them, 200lbs plus of me and 40lbs of bike. AND it had.....rim brakes!!!!!
In all seriousness though, no way would I try and dink (aussie for double up) someone for 20 miles. It'll suck for both of you. Their buttocks will go to sleep as fast as your temper will rise.