Uh, I think a link to a tweet page on the little link toolbar at the top would work.
1.) Person puts in everything for a blog; however, it is under 140 or w/e characters.
2.) Person submits and a message comes up saying something like, “Your blog is too short. If you do not lengthen it, it will be posted in the Tweet (or w/e you call it) section.”
3.) Person makes the decision: lengthen the blog or make it a tweet.
4.) Person goes with latter, blog gets posted into Tweet page, and others can see it and comment on it.
That’s how I would envision it, but I’m not entirely sure how Twitters and its Tweets go.