Air carrier Captain reported misplacing the Operations Agent's copy of the NOTAC. The missing NOTAC was located and delivered unsigned to the Agent in short order; enabling the flight to depart.

Narrative

I received two copies of the NOTAC Hazmat form accompanying the Dispatch Releases and weather package paperwork from the Operations Agent upon arrival at the gate in ZZZ. I believe I signed both copies of the NOTAC Hazmat form along with the on/in report and the Dispatch Release. At this time I was interrupted by an offline jumpseater requesting a ride and stating that he was told he would be required to sit in the cockpit as the flight was full. I coordinated with the A Flight Attendant (FA) to place the jumpseater in the cabin and told him we would notify him if he was needed in the cockpit. I was also slightly distracted with our pre-flight cockpit setup since we were operating with one deferred Control Display Unit (CDU). I bundled the signed paperwork together and secured it using the hat clip next to the jumpseat O2. I took one copy of the NOTAC Hazmat form together with the weather package and stowed it in the card holder next to my right foot. After the Operations Agent collected the paperwork; she returned and stated that the NOTAC Hazmat form was not included. I reached into the weather package paperwork and; indeed; found two NOTAC Hazmat forms one of which I quickly handed to her. I had apparently only signed one of the NOTAC Hazmat forms and handed the Operations Agent the unsigned form. I glanced at the signed NOTAC Hazmat form along with the weather package several times while en route to our destination.[Reporter Suggests]: Immediately address the paperwork to completion before moving on to additional cockpit tasks.