Coils develop spark by creating a magnetic field around thousands of turns of very fine wire, then collapsing the magnetism to create the high voltage. The 12 volt primary winding would have a limited current flow due to high resistance, resulting in a very weak magnetic field. You would get a weak spark, if there was any at all, running the coil on 6 volts.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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