A couple of things. If your wood boiler is set to supply water temperature over 180 degrees, do a parallel piping arrangement because it will trip the limits on the Electro boiler. If you are using an open loop (open to atmosphere) you should use a plate heat exchanger because the excess oxygen isn't good for the Electro boiler. If you're good with both of the first two responses, set up an aquastat to the incoming pipe to the Electro boiler, and it will automatically come on when the wood boiler drops out.