With twice the voltage across a fixed resistance, you'd have twice the current. Not half the current. And if the starter locks up, that's exactly what you'll get.
Starters, like all DC motors, produce a back-EMF proportional to the speed of the starter. As the starter turns faster, less current flows through the windings than when it is stalled or first starting.
So hooking 12 volts to a 6 volt starter results in double the initial starting current. As the motor spins up, the current through the motor drops. The final current depends on the final RPM of the starter, which depends on the load the engine places on it.
The starter will spin faster with 12 volts than with 6 volts. If it spins twice as fast, current will be approximately THE SAME at 12 volts as it was at 6 volts (because the starter is producing roughly twice as much power). If it spins maybe 50 percent faster (typical), the current will be more than half, but less than the same as it was at 6 volts.
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