bafanguy,
The second engine is cross bled started on the taxi-way or apron disconnect point as the LP air connection is located in the airconditioning bays of almost all modern aircraft(notably Boeing`s and Airbus),this connection is located within the engine ingestion hazard zones .So by starting the second engine on the gate using a supplied air source you place all ground staff at risk of ingestion when they attempt to disconnect the LP air source.
Regards DDG