banging sound from gas boiler
Unless you are getting your electricity at a price that is cheaper than the natural gas used in the boiler, I see no need for the electric boiler. I am not familiar with this make of boiler so my writing is on how I feel the units should perform. In the instruction manual they mentioned an optional control that can modulate the gas input from 50% to 100% of the full input that would  or could make the boiler more efficient and save on operating cost. Since I am not physically there, to check the control wiring of the 2 units, I must assume that when the gas boiler is firing the electric boiler is shut down,  if they are wired that way. That being said there should be a device that keeps the pump running long enough to rid the boiler of excess heat. I am assuming that this unit is a coil boiler and those boilers usually  have a flow switch in the boiler's piping to insure water flow before the boiler is allowed to fire and a delay relay to insure that the excess heat is removed before the pump shuts down. You did not mention if this a new installation with a problem that you are observing or if is an older installation with a new problem. More information may be needed to properly give a proper answer. Hope this helps