Quoted from McCune:
How is it a license issue ? Dual coil solenoids have been around since the solenoid was invented and almost every automobile has one.
Williams and Bally had lots of patents on the flipper control circuits:
So if Stern went with a dual-winding design without licensing one of these patents, WMS would probably go after them. I suspect this is why Data East went with the single-winding design in the first place. I'd love to know more of the history behind this.