Unison usually means that two instruments are playing the same notes together. In this case, we have two tracks playing back the same recording - of the phrase "It's gonna rain".
Synchronisation means that things are aligned in time.
And the two channels are just the Left and Right channels in a stereo mix
Listening to the piece on headphones might make things clearer - I think the section being written about starts around 2:30 and ends about 7:30. You can hear the Left and Right stereo channels go out of phase, and then come back into synchronisation again:
Having said that, the particular phrasing "unison synchronization of two channels" is odd. Talking about unison and synchronisation is a bit tautological as the concept of unison includes the concept of being in sync; and arguably it's not so much that the channels themselves that are in sync so much as the snippets of audio that are being repeated.