I would be using two different levels in the same oven.
For even cooking I would swap them half way through. Keep an eye on the internal temps. They are done when they are done. You can't impose a fixed time.
Thanks for the response! Any opinion about searing or not? If yes, what temp and how long? Then how long per pound at what temp???? When measuring the cook time do you use the total combined weight or the weight of the largest roast (24 or 12 pounds0????
If anyone is interested!!! I took the advice of a retired professor at Johnson and Wales University. He was in the Culinary Art college. He advised to forget Searing as that was old school and after rubbing and bringing the 2 roasts up to room temp (about 2 hours) just put into a preheated oven at 325 for about 3 1/2 hours rotating the roasts every hour until thermometer read 130. I did just that and the roasts came out GREAT!!!
Sounds like you hit on a great solution! And certainly easier than searing 2 giant roasts. Glad it turned out well for you and thank you for checking back in.