This is done to VW T4 and Porsche 914 engines.  The company drills and taps a
plug hole from the bottom between the push rod tubes.  The combustion chamber
is reshaped.  You are supplied with a single HI output coil and a special
distributor cap for the 009 that has 8 plugs for wires and a set of 8 plug
wires.  It increases the power in the T4 engine because a lot more of the
fuel mixture is burned at one time.
I considered doing this to my customized 2L in my 73 VW Bus but got a Vanagon
instead and put the money in its engine.

I think that there is a company that does this to T1 heads for racing
applications.  I think it would not work on the WBX head due to the water
passages, But I will think on a way to...

I think that the dual plug systems, one plug fires and micro seconds later
the second plug fires to burn more of the mixture for emissions purposes.

Robert