Do they have the exact same cooking time even though they are different temperatures for the original recipes? Could you look for a different recipe version with more similar temperatures?
If the time is the same and you want to pull them out at the same time, I would cook everything at 375 but put the higher temp item in about 10 to 20 minutes before the lower temp item.
I don't think you can have a simple mathematical formula where you just add or subtract 2 minutes for every 25 degrees because some food types cook longer than others. I think it makes a difference what the two items (size/density) are and if there is liquid in the dish.
A few days ago I was baking pork chops and whole potatoes at the same time and I wasn't going to be available to take one thing out early or start them at different times. I put both in the oven at 350F at the same time. I added some water to the baking dish with the pork chops and covered it with foil. I baked everything for about an hour- ordinarily I would bake the pork chops for about 30 minutes and not add the extra liquid. The pork chops were cooked and not dried out but the water in the dish was pretty much gone. www.ochef.com/878.htm