Each miniExtensions workspace can only connect to one Airtable account at a time.
To connect miniExtensions to bases from multiple Airtable accounts, you can either share all of these bases with one Airtable account that has access to all of them or create multiple miniExtensions workspaces.
You can still add the same Airtable base to multiple miniExtensions workspaces. Each workspace has its own connection to Airtable, so you would just need to add the required bases to each integration!