Yes there is. First of all, lets have a little background on how tags work and what they should be used for. The tagging guidelines can be found on the main meta site, here and they are also explained in the Help Center, here. The following points are relevant to this discussion:
- Tag from the general to the specific. Include at least one broad tag and also include some specific ones.
So, shell for the general and bash and zsh for the specific.
- As a general rule, you should try and add as many tags as you can. You are limited to 5 and, if there are 5 applicable tags, you should use them.
This one is nice and clear. If a question is about both bash and zsh, or if it can be, then it can carry both tags.
- You should try and pick popular tags, those that are used most often.
Again, both bash and zsh are among the more popular tags so it makes sense to include them.
While you should not attempt to summarize the question with the tags, the tags should give an idea of what the question is about. Tags are used to sort questions into categories, so if a question fits into more than one category, it should be tagged with more than one tag. In addition, many users have "Favorite Tags" and questions tagged with them are highlighted. This means that if you use one of those tags, you are more likely to attract the attention of the people who are most capable of answering you.
So, basically, use as many useful tags as are relevant. In the case of the shells, tagging something that applies to both
zsh with bash and zsh makes a lot of sense since i) these are among the most popular tags, many people have favorited them and will therefore notice the question ii) people often search using the specific shell so it makes sense to have the question tagged with both if both apply and iii) there is absolutely no reason not to. You have 5 places for tags, use them!
That said, on the more specific issue you are raising in this question, of course there are examples of questions that need both tags. Please don't try to make such general rules, they are very rarely valid across the board. The tagging of questions should be decided upon on a case by case basis. To take a silly example, if I were to ask "How can I get zsh feature X in bash?" I would, obviously, tag it as both bash and zsh. So, yes, of course there are cases where both tags would be needed.