A simple explanation for the observed effect:
This effect is known as coupled resonance. At the start, the first pendulum disturbs the second pendulum a bit through the string at the top. The two pendulums have the same resonant frequency.(Everything has a resonance frequency.) Energy is transferred efficiently from the swinging first pendulum to the “disturbed” second pendulum. The first pendulum “dies down” after transferring its energy to the second pendulum. Now, the second pendulum “returns the favour” and transfer the energy back to the first pendulum. Then the process repeats by itself.
