Been building up to say this (and sorry, it could have been in reply to anyone of a hundred posters): WE DON'T PLAY 4231 WITH 2 CDMs.
Firstly, if we played with 2 defensive midfielders, it wouldn't be a 4231, but what could only be described as a 42031. Nobody does this. If you play with two defensive midfielders, you don't put four players in your attacking line. Someone's got to take care of the zone between your defensive line and your attackers. If you're the one doing that, you're not a defensive midfielder. If you're a defensive midfielder, you're generally dealing with the zone between your defensive line and your midfield.
We play with 2 central midfielders, which is something different. Though a central pivot can emphasise either the offensive or defensive aspect, or do this differently from game to game, that doesn't mean they're defensive midfielders. McFred is no more a pair of CDMs than Jorginho/Kovacic, Højbjerg/Sissoko, McGinn/Luiz or Bissouma/Allan.