What if any is the advantage of dual plugs in a combustion chamber?
Will it cause a flame front interferance?
If two plugs are better would three be ok?

I remember from my limited flying time that to check the mags you could switch back and forth and then to both and see the RPM change, but....

>>>>>>>>>>>>>>>>>>>>>>>.globle valve

Well there is hemi combustion chambers and semi hemi chambers.  In most true hemis the valves are not at a good angle to allow a small chamber.  Bigger chamber leads to a long burn and, in cases like the Alfa DOHC engines and the Fiat 124 DOHC engines this really inhibits their use as racing engines because you cannot run a really high CR without detonation...the twinplug Alfa GTA's of the 60's corrected the problem with an extra sparkplug (the Fiat had other chamber related issues and needed a LOT more work to be equal in output) Other semihemi DOHC engines of the period, Lotus, Cosworth, etc. Made equal or greater power with only one plug.  Small variations in chamber design can make BIG differences in power output.  I watched a rather large drag race hemi being assembled and it used three plugs.  Big chamber, nitro and, reallllly high rpm.  Three seems to do the job for them.  Oh yeah, I run a twincam Lotus with ~15:1 CR on 104 octane Sunoco with no detonation problems and a single spark plug.

One of the first engines I helped build was for a Cessna T-50 "Crane" in 1958.  When we did the final runup on either engine, the rpm could not vary from left or right mag, or both begore delivery.

"Engines probably weren't very sophisticated in those days.  We had just moved on from flat heads."

Actually, many engines in the beginning of the 20th century were OHV and OHC.  David Dunbar Buick never built anything but OHV engines for the cars that bore his name, only the 29 Marquette used a flathead and it was really a "rebadged" Oldsmobile.

My dad retired from the fire dept.---the twinplug engines were also twin carbed, twin pumped, dual batts...you get the idea.
  
Keep in mind that "high octane" in 1930's was only 67 octane !!!! Oil changes were mandatory every 1000 miles, most cars did not have oil filters and many had no airfilter (valve jobs were common at 20k, too).
Lube jobs came EVERY oil change.  Tires were good for, ummm, 12,000 miles if you were careful. AND...sparkplugs would last, maybe, 8 to 10 thousand miles at most!  Factor in twinplugs and ??????

I learned to drive in a 1937 Nash Ambassador straight 8 with 16 plugs (8 on each side) and a giant 16 plug wire dual coil ign.  I did not really appreaciate the significance then, it just looked cooooool!

So, twinplugs are used in modern engines to modify the flame propagation in a chamber that is a little too big(?), poorly engineered/designed (?)
to modify the exhaust emissions (?) What else?

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