Does it have to be from the input map? you could simply do this:
if Input.is_action_pressed('ui_up') and Input.is_action_pressed("ui_left"):
You can also use the Input Map and assign more than one key, but only one of the keys can be a regular one. The others have to be Shift, Control or Alt, or a combination of them. By long-pressing those keys plus another one you can assign a combination in the Input Map.