The new input system documentation says it's possible to add actions that require input combinations within the editor or code. I only see an option to add one binding path per action within in the Actions UI.
From the documentation:
... require the left trigger to be held and then the A button to be pressed and held for 0.4 seconds?
var action = new InputAction(); action.AddBinding("/<gamepad>/leftTrigger") .CombinedWith("/<gamepad>/buttonSouth", modifiers: "hold(duration=0.4)");
Again, setting this up with the inspector in the editor is an alternative to dealing with the path strings directly.