Carpass
I have a feeling that Bally Heck's reply related to modern twins. The situation with modern four jets e.g. 744, is that the gennies are synchronised and connected in parallel to a Synchronous Bus-bar. The synchronisation is taken care of automatically and the Bus-tie breakers (BTBs) look after the connection/disconnection. The Synch bus itself can be split to allow the two electrical halves of the aircraft to operate independently or can be re-connected to allow full load sharing and (in the case of Autoland) fail safe redundancy for Cat 3 operations. That same synch bus splitting allows the two (unsynchronised) APU gennies (or ground supplies) to power their individual halves of the aircraft without having to be concerned with synchronising.
The actual sychronisation is taken care of by the GCU which is being fed a reference frequency from the #1 BCU. The GCU compares the reference frequency to it's own genny and sends a speed adjustment signal to the IDG. Once the frequency is correct, the GCU allows the BTB to close to connect the individual AC bus to the synch bus.
Have just re-read all that and I'm starting to feel confused.
I think I'd better go and lie down