technically,
the dual sparkplugs allow less ignition timing before top dead center,
which reduces the 'wrong direction' pressure on the piston (called 'negative work')
and leads to slightly better torque and less fuel consumption

in one of the Chrysler histories there was an account of how Chrysler and Daimler engineers got together after the merger and shared previously confidential twin sparkplug research... and both sides admitted that they learned some new effects they had not known before

the gains are real but not large, in the 1 to 3% range

by the way,
the Datsun NapZ was inspired by the original Chrysler Hemi,
then in a turn around,
the NapZ good emissions performance as published in a SAE paper was used to argue successfully for the modern 5.7 Hemi V8