Have you cut the power line to the RX of one of the TZs; You should only ever have one BEC powering an RX, so ideally need to cut the positive wire to the RX from one of them. And the arcing is standard as the caps charge; most people will get this. If it's larger than normal than this is more likely to be a symptom of a problem than a cause. The thing I find that normally blows lemons is reverse voltage; they can't handle even the slightest blip of it where as Lemons don't care as much.. Might be worth looking in to..