When I was in school, we synchronized a generator with the utility power by
connecting a light bulb between them. When the bulb went out, they were in
phase, and we threw the switch. After that, they were self-synchronizing,
because if our generator slowed down, it would start to draw power from the
line, and come back to speed.

But synchronizing two generators in SERIES (instead of parallel) is a whole
nother thing.

Someone asked why you need a X-former. Kuz, if you're running two 120 v
generators in parallel, you getg 120 volts. The transformer gets you up to
240. The OP asked about a way to get 240v out of two generators. He didn't
say he needed two generators to get the required wattage. ONE generator and
a X-former would do it if the power is high enough to run the load.

Take a look at the geneerator panel--Many or most of them have a 240 volt
output, in which case you won't even need the X-former.