New answers tagged safety
Regardless of weight, the advisory height limit of 50 cm will likely be violated if the plan is to carry the two kayaks on their sides. I'd advise against doing so for this reason alone.
Would this be safe? tl;dr: No. It's hard to say exactly how unsafe or how bad the consequences might be. Assuming the rails themselves weigh on the order of 5-10 kg, you're talking about almost doubling the advised load on the whole system. From reading that advisory, it's hard to say exactly what failure modes their mechanical engineers are ...
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