We are just going to go out on a crazy-wacky limb and suggest you cook them both at 350°F for 52.5 minutes. And not a second longer.

much as recipe writers try to scare you, or try to convince you that following their instructions to the letter is essential to a happy and successful life, cooking is seldom rocket science. If you paid serious attention the recipes you read, you would notice that the majority of them specify an oven temperature of 350°F (175°C) – or a moderate oven. Happily, even though your casseroles fall on either side of this benchmark, they both are to be cooked in a moderate oven. It probably wouldn’t matter much if you cooked them both at 325°F or both at 375°F.

The only real issue with egg-based casseroles, though, is that you don’t want them to get tough, which will happen with overcooking. You’ll have to use your judgment as a cook to know when they are done. Cooking two casseroles at the same time consumes only slightly more heat than if you were cooking one casserole – as long as they are roughly the same size. If you cook them at 350°F, you should probably check them first after 50 minutes, and assume they should be thoroughly cooked within 60 or 65 minutes.