As @NickC wrote, there are going to be discrepancies (ie: seat-leon) which will have tags spelled out like this. Other than that, I don't agree with us trying to retag everything with dual tags as you've listed. It doesn't make sense to go through and retag everything, especially when the precedence has been to keep them separate.
There are very few times I've seen where we could use more tags than the five tag allotment we're given. Most times we can easily get rid of something which really doesn't work very well for the question or which there might be a better tag to put in its place. If this was more of an issue, I could see moving in the direction your stating, but as it is, we aren't there. I think we are confusing users of the site as well as creating more work than is needed. It seems very arbitrary, and therefore not needed IMHO.