It depends on your specific use case but I would tie it to the credentials being entered. Hence if User 1 and User 2 have a different distinct account with their own login/password (or whatever) is in use, then they should have their own access token. The reason is that the instance of the app each user is using needs to end up with an access token - so in your example User2 would have to go through the oAuth dance anyway.
The other, more general reason is that it safeguards you for the future when maybe you end up giving different users different rights.
Hope that helps.