I wanted a doorknob for my pax closets and decide to quickly create them self.
I've added one with a M3 & M4 size for the screw.

I also added spacers:
Measure the gap between the doors.
Select the spacers closest to it. 
Jam it between the doors at the height you want your knobs.
With the holes mark where you should drill.

Your knobs should hand 3.5cm from the side and level doing this.