[stevecalifornia]

Pilots also do a 3 handshake exchange when handing over control to the other pilot.

Pilot: "You have the plane." Co-Pilot: "I have the plane." Pilot: "You have the plane."

The third statement is the pilot acknowledging that the transfer of control has completed. Without it, the co-pilot doesn't positively know that the pilot knows that control has been handed over.

As an aside, Air France 447 was a crash where the co-pilot was pulling back on the stick without the captain knowing. The captain couldn't understand why his controls weren't having the intended effect. Both pilots were making inputs, unaware of each other.