Imagine a question fitting into tags z y and x, but you know (and mention) a special case might be answered by a certain field of mathematics which is included in tags w and v and you think people often answering questions in w might know an answer to the question even for more general cases.
Is it okay to tag the question with z, y, x and w to get the attention of people frequently asking questions in w?
If your question would go something like this:
In my investigation of {this} dynamical system I came across this family of complex function $F(z,s,t)=...$ with two parameters $t,s$. I would be interested to know if these functions, or special cases thereof, appears in the literature?
I observed that setting $s=t^2/3=3$ the function is close but not identical to a Dirichlet series associated to a character with conductor $5$. This made me think that such functions might come up in analytic number theory. I have however no expertise in that field so I do not know.
Then I think it is alright, even desirable, that you tag it analytic-number-theory (and/or a more specific tag) in addition to dynamical-systems, complex-analysis or other pertinent tags.
This would be a scenario that Did and hardmath allude to, if I understand them correctly. The 'second' subject is in a way part of the body of the question.
To have the second paragraph is crucial though. You should not just add the tag, and let others figure out why this might be relevant. Generally, a tag should be derived from the content of the question, it should not be used to convey information not in the post.
That is, write in the post that (and why) you think the 'second' subject is relevant and then add the tag. (If ever it turns out this was a misguided idea, you can still edit this out and drop the tag.)
The relevance of the 'second' subject should also be well motivated and intrinsic. To add, say, a tag like algebraic-topology to an elementary group theory question because groups show up in algebraic topology quite a bit and this might get some extra exposure among users following algebraic topology which likely are relatively advanced mathematicians is of course inappropriate.
To sum it up, just explain in the question why you think the 'second' subject is potentially relevant and add the tag to reflect this part of the question. Needless to say, use some common sense and do not add tags gratuitously or on very shaky grounds.
Tags are meant to give an indication of what the question is about. From the Help Center (emphasis added):
A tag is a word or phrase that describes the topic of the question.
They are not supposed to be used solely as bat signals.
So, no, it is not appropriate to give a question the tag analytic-and-algebraic-topology-of-locally-euclidean-metrization-of-infinitely-differentiable-riemannian-manifolds (Боже мой!) just because you think the users active in that tag might be able to answer the question. And I would hope that such uses of tags would be edited out, as they only mess up the signal-to-noise ratio of our tags (already, in my opinion, quite noisy).
