I've yet to hear anyone say that neither fault alone could cause a major earthquake.
If your house topples onto your pumpkin head, it's not going to be useful to you to hear that the earthquake was caused by just one of the faults 'sliding'.
If there is a quake, who's who's fault is it? This reminds me of something a good friend of mine said. He owned a boat. Said if the termites ever stopped holding hands it would sink.