How do I cook two rib roasts (6lbs and 13lbs) in the same oven on different racks?

The oven tends to be hotter the higher inside you go, how much difference depends on your oven and how far apart we are talking about the racks being. If there is no way to have the roasts sitting side by side about half-way inside the oven, then I'd suggest rotating the roasts bottom to top. You can calculate the baking time depending on the size and oven temp, and then do the rotation when the larger one is about half done. Or you could consider cutting your very large roast in half, which will give you three of roughly the same size, and they should cook in about the same amount of time regardless of where they are in the oven. You could still rotate them top to bottom at the half way point to equal out any variance in temp, but it would not be as critical.