is Cooking Two 5-rib Prime Ribs in ONE oven OK? - Page 2
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Cooking time depends on the mass of the item being cooked. More mass/thicker equals longer cooking time.
So your partner is more correct. You have two five pound roasts. Of course everyone's oven is a little different so no one can give exact times but follow the time for the five pound roast and keep an eye on both.