Neither has any followers at this time.
The trouble with this, in my mind, is that the majority of members won't find a tag called "chemical leavening" because they don't think in those terms. And if people can't find the appropriate tag for their question, then they add inappropriate tags instead.
I'm not saying it isn't correct - it is the correct umbrella term. I just don't see any pressing need to merge them or any obvious advantage to using a more generic term. People aren't getting the two confused, and lots of questions really are specific to one or the other (since they aren't actually interchangeable).
We can have chemical-leavening as a tag as well, sort of like we have gelling-agents in addition to gelatin and pectin (interestingly, gelatin is the far more frequently-used tag, which further reinforces my previous point about discoverability). I don't imagine it will get heavy use at this point, though.
I would suggest leaving those two tags alone and manually-retagging as chemical-leavening the ones that are truly about the general concept.