I'm looking for some similar information however I thought I would pass along my typical info on this. There are few fact to keep in mind:
1. You generally can cook anything longer at a lower temperature, but not always faster at a higher temperature (just generally, but tends to be more true in baking). Given that it's usually better to cook at the lower temp and let the off item cook a little longer.
2. In your scenario, I would put them both in at the same time and pull the finished one whenever it is done (Rather than trying to time them to come out at the same time).... Most dishes can (or need to) sit for a bit, just keep it warm with a warmer or cover it up with foil/cover and a few towels or a blanket (more for longer periods)
3. It helps to know how to gage doneness from temp, color, and/or other indicators (clean toothpick, bubbling, etc), it helps if it's a dish you've made before.
4. If the temperatures are very far apart consider using a middle temp or find alternative ways to cook one of the to (I have a toaster oven and microwave oven that make good alternatives).
I just noticed how old this post is, so it may not apply to the original author, but it was one of the first hits in google. I leave it here for others.
In my scenario I'm just flat out trying to cook something faster and trying to find a good estimate for how long a ham that is suppose to take 2.5 hours at 275 will do at a higher temp. Since it is pre-cooked and I'm just heating/glazing it, I don't have to worry much about reaching a specific temp, but I don't want to get it over-done or something. I really need it done in an hour, or 1.5 hours MAX.