OK, the real reason
Simply better power, period, in vehicles that is
If you are a pilot or know one that flys a piston engine aircraft then they can tell you why
Planes have dual spark plug engines, but for two reasons
First is reliability, better to have 2 separate spark systems, just in case, lol
But also better power
On take off BOTH spark systems are activated for best power, once cruising one spark system is shut off for safety, just in case, and on a longer flight they will switch over to other spark system, and go back and forth
And on landing BOTH spark systems are activated, for safety, losing the engine close to the ground is not for the faint of heart, gliding is highly over rated
So it has long been known that dual spark plugs make better power
Vehicles don't need the safety factor just the more power thing
Problem has always been how to do it with a single distributor
Look at this distributor cap:
https://www.partsklassik.com/images/Product/large/2265.jpg
4cyl engine
You also need Dual coils
The rotor is setup so both spark plugs for say #1 get spark at the same time but each spark plug from its own coil
Porsche 911 cap here, 6cyl:
https://www.123ignition.de/ignition/CustomUpload/374O357O340O338O356O369O350O321O365O335O/verteilerkappe-porsche-911-rs-st-rsr-doppelzuendung.jpg
And here:
https://www.aasesales.com/cdn/shop/products/48_972b48a7-7327-41b1-89d5-e87418ad1953_2048x.jpg?v=1571438709
AMAZING stuff, lol, its for extra power, not emissions, MPG or anything else just POWER
Anyway, point being is that Dual spark plugs is better power, I mentioned that right?
So BOTH spark plugs fire at the same time or there would be NO POINT in having dual spark plugs in one cylinder
So disregard the "alternating" thing, obviously they hadn't thought it thru, i.e. what is the point of have dual spark plug and alternating them................so they last longer, lol
With the advent of distributorless spark systems the door was wide open for dual spark plugs, and Ford stepped thru it with the Lima engines
Head space is also an issue, and getting to a 2nd spark plug, V6s and V8s can be hard to machine for a 2nd plug thats easy to get too
Dodge did it on their Hemi's for a few years
But...............4 valves per cylinder, dual overhead cams, 2 intake valves, 2 exhaust valves per cylinder, NO they don't alternate, LMAO, sorry had to take the shot
So the 4 valves made even better power and a better use of head space so dual spark plugs fell out of favour..............except in airplanes
What Ford does use is Waste Spark, very first spark system even invented for gasoline engines
Its crank shaft timed, like magneto spark in your lawn mower, it sparks at every TDC
Much simpler system than cam timed like a distributor
So a spark plug fires once per RPM instead of once every 2 RPM, so a spark is "Wasted" on the exhaust stroke of a 4-stroke engine
But there is no pressure in the cylinder on the exhaust stroke so virtually no wear on the spark plug when it fires
In a Waste Spark system you should only use regular plugs, no coating, OR Double coating plugs, single coating will wear out same as no coating in 1/2 the cylinders