Side loading is a popular option, for both open & closed trailers. Obviously, the length you need will depend on the combined measurements of the 2 vehicles. If a 20' will provide barely enuff room, I'd go with a 22'. And, who knows, you may wanna haul a longer vehicle, later on down the road. So, I'd go with the longer trailer.
http://www.trailersoftheeastcoast.co...-tiedown-rails