I'm not sure about the two shooter theory BUT, I wouldn't rule it out. However, if Sheila was involved, then I think it was a passive role - I don't believe she shot anyone.
I don't think SL would be asking for my help any time soon - then his book would most certainly be a different format!
I gave some serious thought to Sheila also being involved after reading the book, and like you concluded that if she was, it would be in a minor role - unlocking the back door, then being told to wait in the lounge or in her room say, while the first stage kicked off in the main bedroom. If Bamber had relied on his sister firing accurately in half-light, there would have been every chance of his plan going pear-shaped, which it almost did even when he was working alone. And if Sheila was as depressed as it seems she might have been - silent on the trip from London with Colin; sounding like a zombie on the phone; disappointed and upset by her mum and dad wanting the twins to move nearer WHF (if that argument ever took place), plus the drowsiness induced by Haldol, then maybe she might have been more easily persuaded by her brother to end it all and the reason why there was so little resistance from her.
On the other hand, Bamber was probably having a DeFeo moment when telling a tall tale to his prison boyfriend. Or maybe it's just make-believe by the latter.
SL might be considering a volte-face like Bob Woofinden, particularly as Bamber isn't going to preview his book.
