It's completely understandable why Pep doesn't want to go with David and Bernardo as two number 8s or David and Foden or Foden and Bernardo as number 8s. It's because Gundo is more of a typical centre mid who can sit deeper and spread passes from deep. On the other hand, if we play with both Silvas none of them are true centre mids, one is a playmaker who operates in inside channels between the lines (David), the other one is an inverted winger (Bernardo). Even Foden isn't a typical CM, he's more of a copy of David who operates between the lines in inside channels (main difference is physicality, Phil has amazing burst of speed from standing position when he receives the ball and then accelerates). However, neither Bernardo nor David or Phil are true CM's who can lie deep and ping pong passes when circulating the ball from side to side. Pep wants balance (one actual CM who operates from deeper and one sort of playmaker who positions between the lines in inside channels). That's why we see both Silvas starting together in number 8 roles so rarely. When Gundo and KDB play together it's against more physical sides and then it's Gundo who operates more in inside channels on the left.