I reckon that Pep sees those two as completely different types of players.
Walker being the more “defensive” option at RB, where he can stay back on attacks and use his pace to sweep up any counters. When Walker plays we usually end up with three at the back, with him lining up beside the centre backs while the LB goes into midfield.
Cancelo is our right footed equivalent to Mendy. Bombing forward and rarely defending, but still having pace to come back on counters. He’s another outlet for our forward line, his biggest strength being his on-the-ball ability.
So when Cancelo plays, I think Zinchenko will end up being part of the back three and we’ll mostly see Cancelo in the starting eleven if Zin is on the other flank to help keep that defensive line.
You can flip this the other way too with the signing on Angelino. Walker, Stones and Laporte as the back three with Angelino going forward, or Mendy if he’s fit.
2 defensive full backs in Zinchenko and Walker, and 2.5 attacking full backs with Cancelo, Angelino and Mendy.
Then of course if we’re struggling to break down a side we can throw on both Cancelo and Angelino or Mendy with Rodri then dropping deeper to the back three. Using the added width to create more space for the rest of the team. It also lets whichever wingers we use to cut inside and become extra strikers essentially, and I firmly believe Sterling will score 30 goals this season if this theory pans out.