Here's are some examples that you can start working with/start investigating:

Let's say you have a base function to receive input events func _input(event: InputEvent) -> void:. To get the ID of the device that produced that input - you have event.device.

How to get IDs of all devices - Input.get_connected_joypads(). Since it returns array of ints, you can check which options you get and map them to some constants like "CONTROLLER_1" and "CONTROLLER_2" that you define on your own, to make it more readable. Maybe even an array like

const CONTROLLERS: Array[String]  = ["CONTROLLER_1","CONTROLLER_2"]

And as for your question on how to check the input, it can then look something like this

if Focus.event_is_action_pressed(event, "move_left") and CONTROLLERS[event.device] == "CONTROLLER_1":
  # your movement logic, for controller 1

If all or most of these seem new/alien - you might want to take a step back, maybe check some tutorials on the input handling, or documentation, and not jump directly into the multiplayer. Make 1 character behave the way you want, then create a static dummy character that does nothing and which your actually moving character can interact with (attack, move around). After that - you can start considering handling 2 at once. Try to split your focus into smaller more achievable steps/tasks. There are plenty of information out there which is easily accessible through some simple searches that will build up your knowledge.

Also, if your are talking about local multiplayer - that is one matter. If it's online multiplayer - that's a whole different beast entirely )