A phase synchronizer can be quite simple.
For example an LED in a bridge with a simple current-limit circuit can be connected between the same phase of the two generators.
The LED will flicker if they have a slightly different frequency, but as the slave generator is adjusted to the proper frequency and phase, the LED will burn steady, and then go out when the two phases are close together in relative phase (within the threshold voltage of the circuit).
At that point it's generally safe to close the switch between the two generators.
You can use two such circuits for two of the phases, which will then also indicate if the phase sequences match.
If you told us you needed to synchronize two generators together in the beginning we could have saved a lot of wheel spinning here.
Wouldnt it be wise to check the phase rotation of both generators first though?