By default a dual band router will run B,G,N on 2.4 and A,N on 5.
You cannot run G on 5
You have to first disable B and A and then decide if you want N on 2.4. Be nice if you could disable G on 2.4 and run N but there is lots of old equipment around. You want to avoid running g and N on 2.4 unless you really have to. It tends to switch back and forth when it see the other equipment. It can even see equipment that is not yours and do this even if it prevents the other equipment from connecting... just talking to it a little will cause the switching.
If you had only 2 devices one running 2.4g and another running 5n this would be about as good as you can get to maximize the speed though the router.