I hear ya..
But I have the schematics for that chassis right up here and it is a simple series circuit, no doubt about it.
On the Plats . here is what has been found..
The plats have a Plat electrode , but they did not have a Plat case...so the second plugs will tend to foul after a few K miles and they don't idle well b/c of the size of the electrode.
You may have noticed early Fords w/Waste used two different Plats from the factory..one had plat electrode and the other had ground casing ..so , the car actually called for two different plugs and they has to be put in the correct cyliNders for Polarity firing difference...UNREAL.
However, that has been remedied by using the Double Plats. The IR that you use are fine..we just don't like to load the coils R factor b/c the ECU does not handle the extra load well with the series load of 2 plugs firing at once [ large gap ] and the DIS already has Resistors in both the second plug wire and the Plug connectors under each coil. No sense in putting resistors on top of resistors..unless you just like resistors ...
I will check for more info on the possible default of single fire, but all my info and 104 DIS Waste Spark suggest you are incorrect on that point.
It is Series,..check the Schematic 07..001/2 HFM/SFI -104 DIS
If both plugs arced to ground, then both coil towers would have to be Pos+ output, using one to each plug and the engine as Ground. That would make it a Parallel circuit..but the coil has definately one pos and one neg tower, making the circuit SERIES. The flow has to RETURN to the coils NEG [ ground] to complete the circuit.
Can the plug fire using the engine as a ground w/o completeing the series circuit ??? I do not see how b/c the coils secondary is isolated from any grounds... and there is NO center-tap, which would be required for par circuit.... but I will check it out...
As far as the schematic goes , the engine is not being used electrically as a ground, but rather as a connection for the base of the two plugs...and b/c that connection is the threads, the second plug has to fire from case to electrode [ out of phase with the first one]........reversed polarity.
I can see the pos side of coil being possible firing to engine ground b/c engine is grounded neg and the coils pos is looking for a neg ground, but not the neg side.. the coils neg would not be looking for a neg gound , it is looking for a pos to complete....that is where the polarity has to change and that is in the series loop design. AllData states that both gaps have to be in the circuit. We will figure it out, but regardless, the Waste is a series circuit when both are hooked up and the two plugs are out of Phase..that part is certain.