I don't think it matters one bit. Since Jock Scot is absent these days, I'll fill in for him: "You're over-thinking this!" I just really don't think that people put that much thought into it when they see a couple wearing non-matching tartans. Only the nerdiest of tartan nerds would do the mental check to determine what the tartans actually are, and then mentally check their compatibility, and then pass judgment on it.
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