A 2 part canon is a musical passage with a follower and a leader; a theme follows itself in imitation at a certain short interval of time, note for note, most often at the interval of a 4th, 5th, or octave, but can appear at any interval.
A drone (held bass note), or perhaps one or two more moving voices (using "sweet" intervals like 3rds and 6ths which don't interfere with the ear hearing the canon), might be added to help fill out the harmony.
The procedure of working a theme in inverse movement (turning it upside down, interval for interval) and working it as a 2 part canon at the octave with its right side up version following close behind, is a bit more challenging [See blog, Inversions].
It's like flying a jet upside down, with another identical jet trailing close behind right side up, without having a collision.
The 10th variation from Variations Op. 4 is an example of this [See menu bar, Free Stuff].
(con't in Part III)