A lot of the point of being attached by two independent systems (which would normally be two ropes) is about preventing user error becoming terminal as well as protecting about equipment failure. If you totally mess up threading your descender, or thread it while clipped to an gear loop or something, your _entirely separate_ (and different) backup system (i.e. your ASAP normally) will save you.
With SRT you can still operate in a way that protects against user error. For example, you can choose never to use a descender that you haven't first tested while also having a backup (generally a cowstails) - then if you have cocked up loading it somehow (or clipped it to your gear loop) you will fall onto the backup rather than the ground.
When doing a changeover from up to down, once you have unclipped your Croll and sat down on your locked off Stop, slide your hand ascender down a bit but don't remove it until you have unlocked your Stop and tested it.
Always leave a cowstail in a rebelay, when passing it on the way up, for a little bit longer - that way if the anchors have suddenly become knackered (or the rope is over a razor edge or whatever, or just tied with a crap knot or whatever) and will fail in the first few bounces then you are still attached to something!
These are certainly not things people _need_ to do, but we all make mistakes... I have a friend who always used to unclip his cowstail and sit down on his Stop without testing it. He also had a bad habit of not holding the brake rope, for which I abused him solidly for a year. One day he mis-threaded his Stop onto one bobbin only, and sat down at the top of a 40m pitch. Fortunately he had broken the habit of a lifetime and was actually holding the brake rope, so he merely scared the crap out of himself and (in a mild state of panic) managed to clip back in (after dangling in fear for a bit) and do it properly! I was at the bottom just wondering what was taking him so long.
If you find yourself worrying about this sort of thing too much though, you are almost certainly worried about the wrong risks, which often simple things like tripping over and breaking an ankle (particularly on a solo trip) or falling down a dodgy (but unroped) traverse, or slipping on a mud slope, or just getting lost.
