If the screens above are accurate, it would appear that Sonic Classic Collection has ditched the multiplayer modes present in Sonic 2 and Sonic 3. Taken from Sega Europe's Flickr account, the screens don't display the option for multiplayer in either game, unlike the title screens on the original versions of the games. Granted, the multiplayer certainly isn't the main draw of either game, it is reasonably entertaining and would be well suited to the DS' wireless multiplayer abilities. For what it's worth, the GameStop listing for Sonic Classic Collection notes the game is a single-player game.
It's not too surprising that the modes were cut, as it would require additional programming beyond simple emulation of the original games. After all, you can't plug an extra Genesis controller into your DS. Still, it would have been nice to see the multiplayer as a bonus mode outside the games. Just to be sure, we've contacted Sega for the final word on the matter.