I took a Thule Sidewinder, swapped the stock tray for a piece of 6061 aluminum channel (84" x 3" x 1.5"), drilled the channel for the sidewinder (two bolts), drilled the channel for a cross bar mount (two holes), then used a couple bungee straps to secure the rear wheel.

To mount the tandem on the rack, I have my wife stand on the tailgate, I place the rear wheel onto the channel about where it will sit when fully mounted, then she holds it steady as I lift the front of the tandem and drop the front wheel into the tray and stabilizer, then all I do is pivot up the sidearm, strap down the rear wheel, and away we go!

We often use a cam strap from one side of the rack to the other, across the stoker bar, to further stabilize the tandem. We could drive locally without the strap, but it makes me feel more comfortable with the strap in place.

So far we have driven back and forth to local spots and as far as three hours away to Bent Creek, speeds up to 80mph and not a wiggle!

The rack was a leftover from my biking days, I think they can be found for $100-150 used on Ebay. The channel I purchased from On line Metals, $75 delivered.

Here's the rack unmounted, I'll post a pic with the tandem mounted once I remember to take a picture with the tandem mounted