It's always best not to needlessly medicate or over-medicate. A child may well be fine with just Calpol, for example. Or just ibuprofen. And this will be effective. Just the same that it's not healthy to medicate for "just in case", it's equally completely unnecessary to just give both because, well, just to be sure really.
That in itself does not mean they cannot be both given. My interpretation of:
If you give them one of these medicines and they're still distressed before the next dose is due, you could try the other medicine instead.
(which is more or less mirrored in the wording of NHS and nice)
Is that if a dose of one isn't helping, give the other aswel. There's no suggestion of x amount of time between these.
I do agree though that there is no reason to just give both, for no other reason than you have both so you might as well.
I think the guidelines are there to stop people routinely giving both. Maybe at the peak of an illness they will both be needed at the same time. But the recommendations suggest that you don't do this every time. Next does try just one, child might be fine. See how it goes. If needed give the other.
It would be irresponsible for NICE or NHS to encourage use of both as completely normal. That isn't, it's only for when really ill, not for your average sniffle or cough.
That is a different thing to it being dangerous, as an occassional occurance, given responsibly. Which is not the case. As with all medication, it requires responsible thinking. The NHS is required to advice to the lowest common denominator, so has to account for people who cant/won't think responsibly.