How many people are experts on the passive voice, but not the active voice? Or want to learn about one, but not the other? It seems silly to me to treat these as distinct subjects. It would be like having separate tags for "singular" and "plural".
Also, a large proportion of the questions tagged with active-voice are also tagged with passive-voice.
I propose renaming passive-voice to "grammatical voice," and then marking active-voice as a synonym. Of course, keep "passive voice" and "active voice" as redirects.
(Another possible synonym is middle-voice, which only has two questions. However, perhaps middle voice is rare enough that it should be kept as a distinct tag.)