I have to say, that I don’t really understand what you are aiming for but I assume that you mix up what player controllers actually are.
Each character has to be controlled by a player controller so the game know which inputs to use. If you have character A and player controller Z, everything works fine as intended. If you add an additional character B which also uses a player controller Z then this is also fine. Why is this fine? Because a player controller can only possess one character at a time. If you want to test stuff on one machine but with 2 players, you will likely get two windows, one for each player and character. Both windows can use the same player controller without creating problems. The one window moves character A and the other window moves character B. This works because the player controllers have indexes as well.
Link: Get Player Controller | Unreal Engine Documentation
As you can see the player controller needs a number and in computer science stuff starts with 0, so this is the very first player. If have multiple player controllers in one window that you will have index 1 for the second player and so on. But if you want to create a multiplayer experience with multiple windows or on multiple machines, then things change a bit, because every local player is always player controller 0, which might be confusing at first. The number of possessed characters is always the number of player controllers, but you can have characters which are not possessed (like, maybe you want to switch to that one later).
So this is out of the way. What do you want to do? You want to control different characters with one player controller? Like I said each player controller can only possess one character at a time. But you can switch between the characters when you unpossess the first and then possess the next character which you would like to control. I can also think of a solution where you put multiple components in your character and according to the buttonpresses you can determine which component should do what as if you would have a group of something.