How can I supply 240v power to my home with 120v generators?
Two identical 12V car batteries could deliver 24V, and two identical  120V~ transformers at the same net could deliver 240V~.
But two 120V~  generators can not simply be connected in series to get 240V~.
They must have a special interface/circuit to offer that option, because both generators must be locked to exactly the same frequency and must be locked to a constant phase difference of exactly Pi, e.g. by a phase locked loop circuit.
In case of the car batteries and the 2 transformers, this condition is automatically ensured without special circuits. Both car batteries deliver exactly the same frequency (0.000000 Hertz) and can simply be connected with exactly Pi difference ( + pole connected to -).
Both transformers are fed by exactly the same frequency (from the grid) and can also be simply connected with Pi difference.