Hi Ruud! Glad you joined 
By design, Scheduled and Waiting for states are mutually exclusive. This is not a coincidence - those states correspond to GTD concepts. An action can be either scheduled, or delegated, but not both at the same time.
If what you are trying to do is to review some of your Waiting items from time to time, the way to do it is during a daily review process. You can setup something simple like this and it should work: review checklist setup example
Or just schedule a different recurring task to review some of your waiting actions.