I don't know how the stats rate but from the eye test, Cancelo's skill set is different from Walker's. Walker is more physical, speedier, can track back better. Cancelo seems more assured in possession, seems to be a better dribbler, crosser and who offers a different dynamic in attack. If Pep wants to tinker with 3 at the back again, Cancelo is a perfect candidate to play in that role. At Juve and before that, at Inter, Cancelo showed elite dynamism in attack.
If we do get Cancelo, both he and Walker will get ample playing time because they offer a different skill set to the team.