A properly installed parallel system would have valves on the inlet AND outlet piping so you could shut both of them to isolate the heater. Once that is done, NOTHING that happens in the heater can impact the rest of the plumbing. Before restoring it to service, the heater would have to be drainded, flushed, and refilled with fresh water. But, other than standby losses, you only heat the water you use, so leaving both heaters in operation will NOT substantially change, if at all, your utility bill.