If you get either rating you can do difference training to get the other. If you work for a company that has both types you would normally get both at once. When Boeing came out with the 787 they were trying to make it a common type with the 777. It would have saved a lot of training time for pilots already typed on the 777.I don't know whether they succeeded or not as the systems are very different. I got my 757 and 767 ratings at the same time.