Only one of them (Basti) has the role of protecting the defence, the 2nd man in a 2 man midfield is usually either a playmaker or a midfield runner (Fabregas, Ramsey, Yaya Toure etc.)
You're saying Schneiderlin's main responsibility isn't a defensive one?
Schneiderlin's main responsibility is defensive, but it shouldn't be. Schweinsteiger and Carrick are DLP's as MU said, but they should also be screening the defence and allowing the other man to push up. Sr's post stated that 5 of the last 6 matches with this double pivot have ended 0-0, and it's not a coincidence. Players like Mata and Herrera thrive on runs in behind or late runs into the box to get goals, look at Arsenal in 13/14 e.g where Ramsey scored 20-odd goals that season making late runs, the Lampard-Mata combination at Chelsea etc. Schweiny and Carrick are too immobile to be the 2nd man in a double pivot who aren't just playing in front of the CB's, Pirlo operated as the DM and DLP at Juve and for Italy for example, Alonso at Madrid too. We should be playing Herrera alongside Basti or Carrick, or just deploying Schneiderlin in front of the CB's.