Getting the draw reed to sound is the easy part of the overblow, all you need to do is resonate the right pitch. Again I would suggest you start with an isolated reed by blocking of the blow reed. Gently blow and try to find the right tongue/throat position that will allow the draw reed to sound.
It helps if you have a clear idea of what the note you are going to play should sound like. This helps set up the muscles in your throat for resonating the right pitch.
Remember this is not a matter of force, the isolated draw reed will sound at a whisper of a breath if you are resonating correctly.