The responsibility of declaring a split lies with the airline. When making an AMS submission, the airline should indicate that a split has taken place, and HAWB should be included as part of the split.

What is a split shipment?
A split shipment occurs when cargo covered by a single air waybill arrives in the United States on more than one conveyance.

When a carrier splits a consolidated shipment, is it required to send any information for the associated house air waybills?
For consolidated shipments covered by a master air waybill, the carrier is required to report to CBP certain information regarding each associated house air waybill to indicate which house air waybills have been laden on each flight.

For more information visit: https://www.cbp.gov/sites/default/files/documents/air_faq_cargo.pdf