How to define a GameObject for both players?

Hello guys! I'm searching for a while on how it is supposed to work for a given object(which would be a character) to be controlled by 2 players.

The very same thing that happens in a fighting game where both players can select and control the same characte. I have seem something with the PreFab and the tag index (arrays) but I've been facing some problems ... it's ok when I code it very straight forward like:

 if(Input.GetKeyDown(KeyKode.A))//Player one 
 {
    player[0].rigidbody2D.velocity = new Vector2(2f, 0);
 }else 
 if(Input.GetKeyDown(KeyKode.B))// PLayer two
 {
    player[1].rigidbody2D.velocity = new Vector2(2f, 0);
 }

But the problem comes around when I try to put it into a more organized code like the following:

Note: this is found in a baseclass

public void Vel(float X, float Y) {

   if(player[0].tag == "Player")
   }
       player[0]. rigidbody2D.velocity = new Vector2(X, Y);
   }else
   if(player[1].tag == "Player")
   {
         player[1].rigidbody2D.velocity = new Vector2(X, Y);
   }

}

And then call the Vel method in the update method in a child class...

 if(player[0].tag == "Player" && Input.GetKeyDown(KeyCode.B))
 {
    Vel(1f, 0);
 }
 
 if(player[1].tag == "Player" && Input.GetKeyDown(KeyCode.N))
 {
    Vel(1f, 0);
 }

It always trigger the first "if" condition. No matter if I switch to player 2 code it always trigger only the first if. I want to know if there is a specific way to build a multiplayer(offline) controller detection.

Thanks in advance.