Did you check Sampson? They make a rotating kneeboarding rack that is really nice. I think it only holds one due to the thickness of the kneeboarding, but you could get one for each side. I think you could use them in addition to your existing racks. Prepare yourself, they are not cheap.
http://samsonsports.com/tower-access...keboard-racks/
I think they basically machine the center post out of a wakeboard rack.