Title: Why Do Aircraft Engines Have Two Spark Plugs Per Cylinder? | Boldmethod
Headings: Why Do Aircraft Engines Have Two Spark Plugs Per Cylinder?
Why Do Aircraft Engines Have Two Spark Plugs Per Cylinder?
1) Reliability
2) More Power
3) Even Combustion
https://www.flickr.com/photos/scsibug/
Spark Plug Fouling
AA1Car.com
Two Really Is Better Than One
Colin Cutler
Images Courtesy:

Content: This even burn helps prevent hot-spots in the cylinder, and distributes pressure more evenly across the cylinder, which is important with the big, high-compression, air cooled engines found in most airplanes general aviation airplanes. https://www.flickr.com/photos/scsibug/
Spark Plug Fouling
There's one last benefit to a dual spark plug configuration, and it has to do with spark plug fouling. 100LL Avgas contains lead. And to be more specific, it contains "TEL", or tetraethyllead. TEL is added to avgas to prevent engine knocking (also known as detonation). TEL helps prevent engine knock, which is an uncontrolled combustion of pockets of fuel-air mixture in your cylinder, by raising the temperature and pressure where auto-ignition occurs. But while TEL helps with knock, it introduces another problem: lead deposits. If the TEL doesn't completely vaporize in the combustion cycle of your engine, which can happen if your engine is running too cool, lead deposits can accumulate on your spark plugs, causing them to mis-fire. AA1Car.com
That's where the dual ignition comes into play again.