IMO it only makes sense to split a tag for two related series if it meets all of the following criteria:
1) A majority of all questions in the tag pertain to only one version of the series,
2) A significant fraction of the viewers for each version have not watched the other,
3) The volume of questions incoming from both versions is enough that it is not practical for a user to individually check each question,
4) There is an easy way to differentiate the two versions based on tag names, and
5) There are several expert users on that series willing to curate the tags to fix any mis-taggings. …