Almost all the players are getting rotated and those two are the only ones we have for that more defensive role in midfield. As such they've been rotating with each other, which makes perfect sense for most opposition. One plays, one gets rested (and does more intense training), then they swap for the next match.
It'll be interesting to see what he does against Arsenal, as playing both of them may be the best option against top teams. Ugarte is a certainty, so it'll be either Bruno beside him or Casemiro while Bruno plays further forward.