This info is from Sega Dojo:
The man who can’t be stopped (AKA Mojoe) has uncovered even more tidbits of information for the two upcoming Sonic titles, Sonic Adventure 2 for Dreamcast and Sonic the Hedgehog Advance for the Game Boy Advance. What kind of information, you ask? Multiplayer modes, of course!
Sonic Adventure 2
We already knew that Sonic Adventure 2 would have multiplayer games, as we played a few of them at Sega’s booth at E3. However, what we didn’t know is that one of the games would be kart racing! Yes indeed, boys and girls. A new kart racing multiplayer game will be added to the final version. Also, in regards to chaos, collecting them will in fact open up hidden game features and mini-games. Perhaps the chao side quests will be worth it this time around, hmm?
Sonic the Hedgehog Advance
In our fresh-off-the-presses Sonic the Hedgehog Advance screenshots story, we revealed to you a picture where you play as Amy, a character introduce in Sonic Adventure for the Dreamcast. To elaborate on this, Sonic, Tails, Knuckles, and Amy will all be playable in the final game. Could this be the biggest 2D Sonic game yet?
Also, Sonic the Hedgehog Advance will also support multiplayer games between multiple Game Boy Advance. While details on the games haven’t been fully revealed to us yet, we do know that there will be a Chaos Emerald Hunt and Battle mode.
More as we receive it!
Source: Sega Dojo