Cooking more than one meat - will I have to adjust my cooking time?
Yes, you can put both meats into the oven at the same time and take it out at the same time.  However:


You need to sear both before putting them in the oven to keep the juices inside and you will need to cut thin strips from the lamb when serving and then put it back in the oven as it will not be cooked to the bone.
Bake the potatoes before the meat as leaving the meat out for 2 hours will cool it down too much.  Subsequently, re-heat the roasted potatoes for 2 min in the microwave at 750W for 2 minutes, let rest for 1 min and then another minute at 1000W. (or the highest setting) just before serving them.  No one will notice the difference.