Every spring my bike club goes on a camping trip with our bikes; we load 15-20 bikes into a trailer and drive out to the desert. Our protocol is to remove the pedals and turn the handlebars sideways so the bikes will fit next to each other closely, keeping both wheels on. Then we cover each bike with a moving blanket and put a large piece of cardboard (like from a bike box) between each wrapped bike. We don't otherwise bungee them together (it's a tight fit in the trailer), but you might want to consider doing so for your longer trip. So far, we haven't damaged or scratched anyone's bike.