Synchronous (AC) Generators require 5 parameters to match for them to operate synchronously, or in parallel:
Same Terminal Voltage
Same Operating frequency
Same phase angle
Same number of phases
Same phase sequence
The first three are adjustable on the generator in order to get parameters to match for synchronization, the last two are designed and built into each generator and cannot be adjusted without some degree of offline service (number of phases is rather hard to adjust, phase sequence is a bit easier).
If you can’t match all 5 of these parameters, your generators cannot synchronize and you can’t have them working in parallel, that simple.