As for the old style generators, you'd have to mechanically couple the shafts and time the rotor phases together, then synchronize the engine speeds to avoid one dragging the other.
Wouldn't be much fun to do all that and then try to fit them into the camper somewhere.
Id think to sync the generators you'd either have to couple the driveshafts and align the shafts to the same timing, hard to do without seeing how the magnets are positioned.
Non inverter type alternators are easy to synchronise, as long as the units to be be “paralleled” are of identical performance.
Older installations still have manual backup synchronisation that consists simply of lamps situated between the incoming generator (alternator) phases and main bus to indicate if the incoming generator is in phase with the main supply/bus or not.
Basically depending on the arrangement you could tell if the AC waveforms of the incoming and system voltage per phase were matched, and as such synchronised, usually the incoming generators is at a slightly higher rpm so it will take load when connected, so the sync lamps will be slowly fading in and out as you pass through being synchronised and out of sinc.
When generators are connected in parallel the spinning rotors of the alternators are effectively magnetically coupled to each other, no more steps need be taken to keep each unit in phase.
Though the engine Governors and voltage regulators have to be matched/adjusted to ensure equal sharing of power (kw) and current (kvar).
A modern style of manual synchroniser has a ring of leds to indicate the difference in phase angle between incoming generators and the main bus, only one led lights up at a time for a spinning effect, showing the difference in rpm and phase angle, you adjust rpm to have a slow clockwise rotation and connect when the top led indicates synchronisation.
I’m not really recommending it be done, but matching domestic non inverter generators can be diy paralleled without much trouble, the most likely problem would be if the voltage regulators didn’t have the same characteristics, this would lead to one of the alternators taking more that it’s share of current, even if the driving engines have their loads equally matched (by the governors), would be best to use a cheap eBay power meter to monitor each alternators output.
