Other people have suggested Bamber was wrong to go ahead with the murder/suicide plan after Sheila's two shots.
Some people are now suggesting that Jeremy may have read about two shot suicides during his preparations, seemingly anticipating that he may use two shots, and presumably making sure that this eventuality would not hinder his plans.
Luckily for him he must have discovered that two shot suicides are indeed possible and as it turned out, coincidentally, he did use two shots but armed with his foreknowledge about unlikely suicides he was able to carry on as planned.
What amazing foresight Jeremy had!!