If the Cubs get both Jackson and Villanueva, it'll be interesting to see what happens next.
We know Garza, Samardzija, and Jackson would fill the first three spots in the rotation. Everything we've heard from the front office makes it sound like Baker and Feldman are being looked at as starters only as well. So that would leave Wood and Villanueva in the pen.
BUT...given Villanueva's strong preference for starting (it was said earlier in the offseason that he would only sign with a team that was going to let him start), it seems unlikely that he'd sign somewhere that didn't guarantee him at least a shot in spring training to compete for a rotation spot. So, I would think that signing both of them would mean at least one of two things: (1) they expect enough problems with Garza's or Baker's rehab that they'd have at least one spot open for competition in spring training, or (2) there is more trade interest in Garza than we'd expect despite his injury.
Chances are, the Cubs will miss out on one or both of them, though. So we probably don't have to worry about any of this.