So if you go to your Developer Dashboard and choose “Sandbox”, you’ll only see one access token. This access token is tied to your “Default Test Account”. In the API explorer, this is the access token that appears in the drop down (for each application, the access tokens are all “Default Test Account”).
Now, if you go to your apps page (https://developer.squareup.com/apps), if you click on any of the other sandbox accounts, you’ll see authorizations there which have different access tokens. These can be automatically generated when you create one by leaving the “Create authorizations for all apps” option checked. Alternatively, you can go through the OAuth process to generate an access token for any of your sandbox accounts. An access token is always scoped to a single account, but many applications can have a different access token for the same account.