First, buy a fork mount that you would normally use for a bed of a truck.
Then, sandwich the cross bars of your bicycle rack with two 4.5 inch 1x3 pieces of plywood. You'll drill a 1/4 inch size hole in all four corners, run your bolts through them, and screw on your nuts.
You'll also want to mark and drill for each hole on the fork mount. When you do that, you'll run two more bolts and tighten down your mount. It's ok if the mount rests on the nuts as long as it's even
You will then remove the front wheel of the bike you're trailing and be sure to make sure the mount is tightened down snug.
You'll then take two bungee cables and hook them from your rear rack to the rear drop outs on the bike you're trailing. This is for stability. I would also bungee your front wheel to the rack