Well they may sell a switching device that connects the batteries in series to put 24v to the starter, and the starter MAY handle it for short bursts for a little while.

BUT, I'll also bet you they put all kinds of disclaimers on their device saying they aren't responsible for any damage to your car or its electrical system/components. And if you are putting 24 volts on a device (starter) designed for 12 volts you WILL burn it up. Maybe not the first, or even fiftieth time, but it will happen.

Volts / Resistance = Amps and Volts x Amps = Watts. The starter windings are sized for 12 volts and have a certain resistance. Doubling the volts means twice the amps. Twice the amps AND twice the volts means FOUR times the watts. That power has to go (be dissapated) somewhere and where a lot if it goes is HEAT. Heat in the windings of a motor breaks down the insulation on the wire - and the winding wire is only insulated with a thin coating of "varnish" (plastic like coating) in the first place. Heat can cook the varnish off the windings causing a short.

Since the insulation on the windings is designed to withstand 12 volts and you are putting 24 volts to it, any thin or weak spots in the insulation are twice as likely to burn through and become a break in the insulation - again resulting in a short. Needless to say, a short in the windings equals a dead starter. Unfortunately, it only takes seconds for the starter to get really hot when cranking, even when connected to 12 volts, but it takes several minutes for one to cool down. Connected to 24 volts it will get hot four times as fast - or four times hotter in the same amount of time (remember: four times the Wattage), but won't cool down any faster.

So, while you may be able to get away with hitting the starter motor with double the voltage it was designed for, it is a crap shoot and you won't get away with it for long. It may work, or it may fry the starter the first time, but even if it doesn't, sooner or later you're going to crank it too long, or too many times without sufficient cool-down time, and fry your starter. I know I wouldn't do it except in an emergency - certainly not as a regular means of starting my car. Unless of course you happen to have several spare starters and like changing them

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown