Step 3: Rear bike
this drilling is going to prevent the frame of the rear bike from turning like the cargo of a semi-truck, which could be catastrophic if it happens on a left turn for the drivetrain.
MAKE SURE THAT WHEN YOU DRILL, THE FORK IS LINED UP PERFECTLY STRAIGHT WITH THE FRAME. I don't think I need to explain why. it might be easier to check this with a front wheel mounted, or if you particularly don't care about it, clamp it in a vice as I did.