I doubt this would increase the number of tanks at all. I also don’t think it’s even a good solution to almost anything.
Dual Spec is an anti-feature that would actually cause more harm (by harm, I mean change, difference, divergence from normal) to TBC, and create a different meta, as well as benefit some classes greatly, while not benefiting some other much or at all, and even acting as a detriment to some players of some classes…
I do happen to have some opinions on this matter though, and some ideas on what I think would be a better solution to some problems that have definitions…