cooking 2 items at different temps at the same time. 375 for 20 min. 400 for l5 min.
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Lisanne is a trusted home cook.
Depending on what you are cooking, it probably shouldn't make a great deal of difference; set the oven at 375 for both if you wish to cook them together. If you let us know what exactly you're cooking (or better yet, link to the recipes) we can give a more accurate answer).
What is the temperature that you want your food to be when it is finished? That is the factor. Not time. Get a got thermometer.
That temperature difference won't make much difference, but if one of the items is something like bread or cake that's active in the oven, I'd use the recommended temp. for that; meats, vegetables etc. are more flexible.