Q:

Word order: Neck & Back Pain VS Back & Neck Pain

I'm working on a website and I'm trying to figure out which one is correct:

Neck & Back Pain

or

Back & Neck Pain

A:

Both are correct.
As to which is more common, Google Books Ngram Viewer suggests that back and neck pain has a slight lead in popularity—at least with the written form:

If I look only at the UK English corpus, back and neck pain is much more common:

If I look only at the US English corpus, the two are essentially identical—even though neck and back pain has a very slight edge:

Finally, if I look at the 2009 US English corpus (which even, I think, Google says is questionable), it indicates that the lead for neck and back pain has increased:

(The data for 2009 UK English shows no difference from the normal 2000 UK English.)

On the other hand, it also shows that, taken singly, it's far more common for back pain to appear in writing than it is for neck pain. (That link is for US English, but it applies to both US and UK and to all years.)
In short, take your pick. The combined back and neck pain would likely sound more normal to poeple in a global audience. But since you tagged your question with american-english specifically, this data is at least suggestive of the fact that neck and back pain is a bit more common. Still, both are easily understood—and you should simply pick whichever sounds best to you or fits in consistently with other phrasing or stylistic branding that you've used.

