FGN has the following news on the capabilities of this new Link-Up thing with Sonic Adventure 2 Battle and Sonic Advance:
The titles can exchange data between them which can be played separately. The core idea is based around the artificial life form – ‘Chao’, who can be brought up and transferred between the two games. They change both appearance and personality in accordance with the way they are bought up. A demonstration of the link-up capabilities is planned for the Tokyo Game Show 2001 Autumn.
Hmmm…
Source: FGN Online