TL;DR - The pilots failed to engage in a 'positive exchange of flight controls' procedure and ended up providing opposite control inputs at the same time, so it felt to both of them like the controls were unresponsive / jammed.

This is a well understood and generic problem in dual-control aviation - see https://www.faasafety.gov/files/helpcontent/Courses/ALC-36/c... for example. The standard exchange of flight control procedure is designed to create a common understanding between the pilots as to who is in control of the plane. Interestingly it's similar to a TCP three-way handshake:

Pilot A: You have control of the plane.

Pilot B: I have control of the plane.

Pilot A: Acknowledge you have control of the plane.