I have a single oven. My ham needs to cook for 2 1/2 hours at 275 °F, my scalloped potatoes need to be cooked at 400 °F for 1 hour.
Do I put both dishes in the oven at 275 °F and if yes, when do I take my potatoes out?
Timing can be more challenging than creating a meal itself. When this combo is part of a family gathering, I take timing out of the mix by baking the scalloped potatoes first, cover and set aside. Then do your ham. While the ham “rests”, to redistribute the juices, put the potatoes back in the oven, turn oven down to 300F & warm them. Good luck!
Browning will not occur at 275 even if you leave the potatoes in the oven for days. So if you are happy with soft, creamy, pale potatoes, your method will work. But if you want a nice brown crust, you'll have to take out the ham and turn the oven up to 375 or more for the last 15 minutes or so of cooking.
Scalloped potatoes will cook in 3 hours at 275°. I have done it before and there are recipes that call for that time and temperature.