The 727 had parallelling, but the crew had to do the frequency adjust manually (if I remember well, 727 too long ago).
The 777 again has parallelling during power breaks. It occurs when you swith power sources between external, APU and generator power on ground, to avoid the flight deck screens go dark and other system loose power for a moment.
A reference frequency circuit is in the BPCU, and for a moment two sources are parallelled. The generator controllers adjust IDG speed to match power of the other source, for the APU it's the APU RPM that is adjusted through the APU controller.