Start by checking the shooter lane switch for loose wire or diode disconnected or touching where it shouldn't be (if it has a diode). If that doesn't check out go into switch mode and see if it sees the switch activate or is stuck on in switch mode. If it does activate in switch mode you'll need to dig deeper. If it doesn't then you can actually use a probe put in buzz mode to check for continuity, put it across the switch and activate it to see if your meter buzzes. If it buzzes switch is good, dig deeper. If it doesn't buzz you may need a new switch.

But my general feeling here is the switch isn't getting a signal to the board letting it know the ball has kicked out so it kicks out the second one.

There you go!

Yes if you are checking switches with your fingers you need to check them by using the ball to trigger them instead. Might find the fault that way. The issue is like in one of the switches in the trough or shooter lane. Probably trough.

If cleaning helped the yes that's the culprit. When I have troublesome switches and I clean them and 5he problem either doesn't completely go away or returns I just replace the switch. That's on my personal games. I'm getting to the point on location I just replace the switch initially and be done with it.