They pair of them really are splitting the duties 50/50. Really good partnership so far.
There's 2 main types of holding midfielder. The combative, less technically gifted type (Makelele, Lennon, Carsley). And the deep lying playmaker (Pirlo, Alonso, Xavi).
imo the main job of a holding midfielder is to always be in the best possible position to receive a pass. That's it. He doesn't need to be a great tackler, Pirlo can hardly tackle. Too many people think of the holding midfielder as the player who's main duties are defensive. Gattuso's main work is defensive and he's not a holding midfielder. Sissoko is similar.