Older Williams machines that took two coins for a single play used a
mechanical method for counting the two coins. This is different from other
machines which used an electromechanical method, usually a mini-stepping
unit to count the coins.
The Williams method uses a little flip-flop diverter, called a 'flip-over
device', next to the coin switch. For a single or multiple plays per coin
the diverter is blocked from flipping, staying in one position and the coins
slip through as normal tripping the coin switch. For two coins per play
the diverter is first positioned so that the coin slips past away from the
coin switch into the coin box, causing the diverter to flip to allow the
next coin to hit the switch. When the next coin trips the coin switch the
diverter flips back to the first position. A good diagram of how this works
is in the Williams manuals for most games that used this device.