Microsoft and Activision had no problem when Modern Warfare and Halo 3 released the same year. Halo 3 released September and Modern Warfare released November. Both had incredible sales and so did Assasins Creed. As for advertising MS would gladly boost advertising $ to pitch both games.
Think about what better way to tell hardcore gamers they are not being abandoned for casuals with Natal. MS point blanks says all types of gamers will be taken care of.
Its libraries that sell systems not a single game.