Why 2 DEs? Because the only other DE we have on the roster behind Jared and Tamba is Hicks. Who is old, slow and doesn't really provide much outside of solid run defense. If Jared or Tamba get hurt and have to miss time, we're kinda screwed. Wilkerson is a tweener but seemed to show a little more lining up inside than outside. But he can play the outside in a pinch. But we really don't have any true depth behind Jared and Tamba. And like I said, if either one has to miss time (like Jared may have to in the beginning of the season) we're kinda screwed as the roster stands right now. That's why I'd go after 2 DEs.
As far as DTs go, that's why as part of my trades I was having us get Haynesworth from the Titans and Manny Wright (or Rod Wright) from the the Phins.
I think the Chiefs are eventually going to sign either Mitchell or Harris. Probably whichever one is cheapest.