When i was a kid my grandma bot my cousins a copy of generations the reason she bought it was because the had no local split screen games and she saw to sonics on the front and thought "multiplayer" and got it for them for christmas why have to sonics and no co op where two players control them both at the same time? :(
Sadly, only the 3DS version of the game has multiplayer.
Also, the two Sonics play totally different, not to mention the stages which are designed for two different styles of gameplay.
if you put two characters on the front i feel like it should have multiplayer like sonic the hedgehog 2