Many people do carry bikes on a rear hitch on the back of the 5er (or TT). But if you have ever followed one down the highway you will see that they do bounce a LOT.
While towing the 5er I have carried 3 26" bikes in the bed of my long bed truck in front of the hitch by removing the front wheels and laying them flat in the bed, and a childs bike laying flat behind the hitch. With folding bikes I would think you could do that.
But getting tired of removing the wheels, I made a pin box hitch mount to carry them on the front of the 5er. Much easier.