Playing Masch in a double pivot would negatively impact our possession game, but I think there are 3 hidden advantages that haven't been mentioned:
1) Par Xavi, Busquets is our best distributor. As sole DM he can't really assume that role, but with Masch in midfield – he can. In fact he can still act as our main man in buildups, at the cost of playing as a deep lying playmaker which given his limitations going forward wouldn't be a cost at all.
2) Ceding possession more often means that now the other team has the ball and needs to move it forward or risk being dispossessed in their own half. So if that team is parking the bus, they now need to move it. Against a powerful counter attacking team like RM, it could be even more effective since they would be more willing to move forward creating space for us, and by having to make more frequent sprints they would end up getting tired faster.
3) A double-pivot would be ideal for containing Bale/Christiano. By dropping Masch back as a 3rd CB when we are in possession we gain the ability of being able to either double mark B/C or have Alves/Alba push forward without leaving space behind.
IMO, a 3-4-3 with a double pivot would be the ideal formation for us; but to make it work you need at least 2 CB who have pace in abundance and now we have them (Mathieu/V); although I wouldn't recommend taking such a radical step less than a week ahead of a Clasico.