@Wibble
I don't understand your point.
With side mount you have a similar problem to that of Independent twins.
If you loose the first/second stage on one of the cylinders, you can no longer access the gas in that cylinder. Thats a huge black mark for me. Been caught once, have no interest in being caught a second time - I might not be so lucky a second time!
I tend not to dive tight squeezes, so I don't need to use side mount. In the past I've passed a twinset through a hole and followed it. But I haven't dived anything in recent years which would require that tight a squeeze. As a general rule, if it's that tight, I'm not going there.
90% of my dives this year have involved carrying at least one stage. It's a real pleasure when i can get rid of the cylinders from under my arms. I bloody hate carrying stages.