A question with just a required tag isn't really a tagged question. discussion doesn't really explain what topics the discussion is about... it's basically a meta tag. A useful meta tag, but quite not enough tagging.
Well, I've posted questions with just the discussion tag on MSU before. In fact, a good chunk of them just have one tag, since its all I need - if its to do with general site policy, or what we need to do with a single specific tag. I don't see either of them being any clearer with an additional tag, personally.
I've also used specific additional tags when I've wanted to do something that didn't quite fit the way things were usually done, and the second tag would help others find it
I really think this is yet another one of these things I feel should be organic to the site - it isn't a major thing to fix, and in quite a few cases, the additional tag might be somewhat localised. I don't really think there's any advantage to having a compulsary need for an additional, non mandatory tag