First forget about playing the overblow and just concentrate on playing the blow reed. Slowly bend down the note and try to reach the point where the blow reed chokes.
Chances are that you when you do reach this point the draw reed will immediately start to sound.
If it doesn't then don't panic, stay concentrated on keeping the blow reed choked and gently try to get the vocal tract in the right position for resonating the note the draw reed should produce.
If this turns out to be very difficult then try to set the gaps on both reeds a little lower (or very low) that should make them respond better.
|Blow to choke to overblow|