I bet it would be a fairly simple junkyard build. Start with a trailer. Replace the rear axle with one from a rear-wheel drive car. Make your own driveshaft for it, with sprockets for the bike chains. All the cranks would need to be freewheeling. Would just need to figure out a way to reverse the rotation direction for the cranks on the opposite side.
Just wanted to add this photo as an alternative to those who prefer to travel solo.
Pretty clever trailer design. Also a kayaker. With suspension forks, I could load my boat with all its gear and turn a few heads towing it to the river on my MB. Now all we need is a hotdog cart!