The Iranian nuclear deal is not being negotiated between two heads of state.

The US and Iran have no formal diplomatic ties and there is no such thing as formal meetings or negotiations between the US and Iran.

There are seven states involved in the Joint Plan of Action for an agreement on Iranian nuclear program - Iran, the US, Russia, China, France, the United Kingdom and Germany. It's known as Iran and the P5+1. In the EU media it's often known as the E3+3.

Now going back in history, there were alternative negotiations during the US Civil War by the opposition party/factions who sought a quicker end to the war