Fake. First of all you would need two people: one for negative and one for positive. Second the electrical resistance of the human body is far to high. At 12V you can only barely light up an LED, let alone run a starter. Dry skin resistance can be as much as 100kR if wet it can go down to 1kR. That would mean best case scenario 12V/1000R=0.012A or 12mA. A starter needs several hundred amps to run. Therefore the voltage across your hands would drop to zero as the resistance is too high.

Also this fake trick isn’t even dangerous. 12V is SELV. You’d need over 120V DC for stuff to get dangerous. Though you can already feel a slight tickle at ~50-60V DC.