I have a different cart with bunks that run parallel to the thwarts (when the canoe is atop). I think it was designed as a kayak cart. As others have mentioned, I have experienced a lot of twisting and have had to run lines from the bow/stern as well as use both belly straps across the mid-section of the canoe to control the twisting.
Recently, however, I realized I was using the belly straps wrong and seeing Glenn's pictures, where each strap comes across the top of the canoe as a single stand, reminded me of this.
Instead of running each strap as a single strand (either completely under the cart or attaching one end to one side and pulling a single stand over the cart to the other side), try taking a strap around some part of the frame on one side of the of the cart and pulling BOTH ends of the strap back over the top of the cart before feeding the plain end of the strap around some part of the cart frame on the other side of the canoe and brining it back through the cam. This gives you two strands across the top of the canoe per strap.
This is the way I was taught do it when car topping the canoe (although the canoe is upside down on top of the car and right side up on the cart). I can't explain why this approach is more secure, however, I have found this arrangement much, much less prone to twisting on the cart as compared to running each strap as a single strand.