I noticed that we have a nes tag but do not have a famicom tag. I recently asked a Famicom question (What is the Famicom "Pulse Line" of games? How many games are part of this line?) and used the nes tag instead of creating a new famicom tag.
I think ideally you'd want three tags, one for things that cover both, and separate ones for things that are specific to each console so that you can easily search for information specific to one console or the other. I'm not sure how workable that would be though. Having famicom, nes, and famicom-nes, or something simialr, could easily get very confusing.