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Whisk together the vinegar, lemon juice, garlic and thyme in a large bowl.
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Season with salt and pepper and gradually whisk in the olive oil.
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Slice both of the squash diagonally into 1/4" thick slices.
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Spoon 3 tablespoons of the marinade into a small bowl, cover and set aside.
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Add all the squash to the remaining marinade in the large bowl and toss to coat.
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Transfer the mixture to a 13x9 glass baking dish, cover and marinate at room temperature for at least 3 hours.
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Can also be refrigerated for up to one day to marinate.
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Season squash with a little additional salt and pepper and grill on the barbecue or on an indoor grill pan until they are crisp tender and brown, turning occasionally for about 8 minutes.
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Transfer squash to a serving platter and drizzle with the reserved 3 tablespoons of marinade.
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Serve hot or at room temperature.