I experience something similar while carrying two ww playboats on top of my car
I am lucky - both boats have safety grab loops, I thread straps through them, cinch it down, straps stay.
Perhaps something similar you could rig up - using end grab loops, intermediate strap between them. The intermediate strap wouldn't be meant to hold the boats, but provide tension for the bow and stern rack straps?
PFD's (Life Jackets)
Overstock Outlet Foods
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