A twin plug system can be very effective in certain combustion chambers.
Essentially, it is a "band aid" for a chamber that is having flame travel issues.
This is common on hemi engines with high compression piston-domes which divide the chamber. Or on other chambers with excessive flame travel or impediments, or insufficient "squish" area.
It can help an older engine design remain viable and gain some power with less emissions, when there is a need to keep the engine in production.
I'm not quite sure why anyone would design a modern chamber which needed twin plugs to work correctly, when it is quite easily accomplished with a single plug design if you do it right.