If there is a minimum width for the Hullavator but if there is, Thule will be able to tell you. I have the Hullavator on my VW GTI and the racks (front and rear) are exactly 28" apart and the rack, Hullavator, and my Oasis 14'6" are rock solid.
I'm not sure if that is what you mean by minimum width, but I do know that you cAn have Hullavators on both side of your vehicle. My dealer suggested, and I got 58" crossbar's which allow proper clearance from the Hullavator cradles from the car when lowered. I could also put another Hullavator on the drivers side if I had the need.
Hope that helps.
2011 Oasis Ivory Dune