I think these combos are too similar.
But evenso what has Zinchenko have to show playing Cancelo or anyone else for that matter playing LB is not an improvement.
All 4 have different strengths.
When Cancelo and Zinch play, Cancelo attacks and joins the front line on the right, while Zinch is more conservative and great with possession and Defensive attention.
The other combo, Kyle is more like Zinch, albeit with less technique and more Speed and Athletisn, while Mendy is more attack minded vans perhaps the best crosser.
Cancelo as great as he is actually loses possession a lot, so he needs the most possession friendly fullback, Zinch, as his foil. And Mendy is the most attack minded, thus needs the best defensive back, Kyle as his foil. Also Kyle has the meanest Cross field pass which has its highest utility when Mendy is the cross field fullback.
I've considered some of the qualities all 4 players possess, and I'm strongly convinced that a prime Cancelo/Zinchencko and Walker/Mendy are the 2 best pairs.
Granted with Walker/Mendy we lose some possession edge, and with the other some Athletisn and directness.