my opinion is less than educated. 
Me too.
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I would imagine that with the chains both lined up, the "force" would be transmitted through the heavier inner plates of the chains and then the lighter outer links of both of the chains. Having the lighter links lined up "may" put a weak spot in the chain.
With the chains staggered, the strength of the chain would be more equal with 1 chain being on the lighter outer links when the other chain is on the heavier inner links and on and on.
I'm not a scientist, but it makes sense to me.