Timing reading doubled with a waste spark set up

Why does a waste spark system give an incorrect reading when checking the ignition timing with a timing light?

I have been told to halve the number or if available, use a 2 stroke timing light

Waste spark fires twice the amount of time of sequential ignition. On ignition and exhaust strokes hence the incorrect reading.

To be absolutely certain the timing is accurate, often you're better off using a simple timing light without a dial back function. This way you'll get two flashes per engine cycle but they both occur at the same point on the crank pulley so appear as one.