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?
Kayak & Canoe Outriggers
The Kayak Wing
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