I have one of these things I think, maybe just the older version:
https://www.thule.com/en-us/us/bike-...-535-_-1683514
I have it mounted as far out as I can before the crossbars begin to taper (mine aren't adjustable, maybe yours look slightly different). With it mounted here I can fit my 8' 6" surfboard, but I'm not sure how a SUP would fit, especially if you are trying to get 2 bikes up there with it. Overall I like the Thule mount but it has loosened up some after being mounted and unmounted over the past couple of years, so now I just leave it on there. If this one ever loosens up to the point that it's not working, I think I would go with the Rhino Racks fork mount.