Having two is interesting. You probably couldn't have asked a more polarizing and controversial question. Ground planes serve two purposes, 1 to add a means to shunt leaking high voltage to ground just like the ground in a home electrical system is attached to a water pipe and allows a dead short causing a breaker to trip thus protecting people aboard, but not so much for swimmers. 2 creates a low voltage equilibrium between all bonded metal in the water and within the boat and thus causing the attached zinc anode to corrode as opposed to the prop. Only you can see what condition the grounding and bonding systems are in and how they are wired up. do you need them? It depends on how you use the boat. You can immense them in a bucket of phosphoric acid and they will look as good as new in a few hours, they are pure copper after all. you might want to replace the bolts though. the bolts are bronze but one bolt of the bunch is gold plated to keep a nice strong electrical connection to the wiring and copper plate.
