Lots of things do not follow what is commonly considered ohms law, semiconductors for one. On a microscopic level maybe yes but as a discrete component, then no. You have to be careful when applying ohms law to things other than pure resistors.
Another issue is that "power" is several things in a motor. It is electrical power, amps times volts, but it is also horse power, torque and shaft speed.
Initially the motor is not turning and the resistance of the wire is the same for 6 or 12 volts. No back EMF since the motor is not turning. At that point the current doubles when going from 6 to 12 volts and the torque doubles. The watts also increase by 4 times if the battery can supply the needed current at 12 volts. The HP does not change at this point since it is zero for both, the shaft is not turning. This is the full stall torque. Once the motor starts turning then the current will decrease due to the back emf. At some point the motor will reach max wattage which will be close to 4 times the 6 volt wattage, but wattage and HP are not the same. Friction, back emf, ability for the battery to supply both 12 volts and the needed current, etc...
Still, I would expect the HP to be close to 4 times the amount possible with 6 v, some losses due to heat losses, friction and so on.
What does this have to do with breaking teeth on the starter or ring gear? The initial torque applied to the gear teeth is twice as much at 12 volts vs 6 volts. The power, either electrical watts or mechanical HP does not really matter since the starter shaft and gear are still not spinning very fast. At this point it is the torque that is important. The strength of the magnetic field and therefore the torque comes from the current through the windings not the voltage. The voltage is important since it will determine indirectly the current since the windings have some real world resistance.
What is breaking the gear teeth? I have no idea. There is more torque and maybe that is enough but I would not think so. The gear is being wound down the spiral shaft to the ring gear with more force on a 6v starter being run on 12 volts. Maybe if the teeth are unfortunate enough to hit dead on each other that could place enough shear forces on the teeth that they break.
I have not had a problem with teeth breaking on 12 volt conversions but for those that have had a problem then slowing the starter with a smaller battery or thinner cable may work for them.
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