The dual plugs would fire large amounts of EGR diluted and hard to light lean mixtures.
An added plus is the need to reduce your timing advance. With two points of ignition the fuel burns sooner. Shorter burn time means lighting it later is possible and still get maximum cylinder pressures just after TDC . This means less time at maximum temps and pressure when NOx is formed. (the real reason Nissan went with it)
Shorter burn time means less time for heat to be transferred to the combustion chamber walls and piston tops and more available for expansion pressure to push the pistons down. This would reduce hot spots and pinging on an already detonation resistant hemi head anyway.
In N America the early Z20 single or dual plug head is the same as the Z22 heads which were all dual plug. The Z24 heads are the same as the Z22 with these exceptions.... The opening for the mechanical fuel pump is not machined out so no block off plate and the intake ports are squarish with rounded corners rather than round.
All Z20/22/24 combustion chambers are 57cc, all valve the same size and all cams the same.
In '83 when the Z24 came out in the 720 truck a 'Mileage Option' Z20 engine was offered. It has smaller combustion chambers of about 45cc and flat top pistons for 9 to 1 compression (and dual plugs) Note the small top and bottom quench areas on the W04 casting Mileage Option Z20S head above
Dual plugs would be more detonation resistant and more efficient. On a stock engine the difference would be small. On a turbo high HP engine this would be magnified..