That's a good one. I guess I would look at the ground wires. If one of the grounds is not making a good connection the current for both lamps may be flowing through the one lamp. I'd have to look at the wiring to be sure if that can happen but current will try to find a path to ground and it may not be where you want it to go.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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