It is generally easier to memorize groups of notes, or patterns of notes, as opposed to memorizing each note individually.
In passages like the ones shown, it can be helpful to "simplify" or "strip down" the passage to essential elements. This sometimes makes clear the composer's intention, which may otherwise be obscured by the "extra" notes. It also can facilitate memorization by simplifying the structure of the passage.
Applied to Clementi
In the case of the Clementi, notice that each beat of sixteenths comprises two different pitches. So one way to practice is to turn the pitch pairs into chords, and hold the chord for the expected duration.
What now becomes more clear is that there are three voices: two in the right hand, one in the left. In this case, another practice technique is to play each voice single, then in pairs, and finally all together. Below is a "hyper-simplified" version. The practice technique goes like this:
- Right hand plays top-most voice, left hand plays middle voice.
- Right hand plays middle voice, left hand plays bass voice.
- Right hand plays top-most voice, left hand plays bass voice.
Applied to Bach
With the Bach, the key observation is that the 32nd notes are "decorative". Although not technically ornaments, they serve a similar purpose. The main note occurs on the beat.
Just looking at the reduced version suggests that each hand is, in effect, playing a three-beat chord in each measure. These could certainly be practiced hands separately, and might work hands together as well. That is, both hands together might form a single chord. (Hint: They do, except the second measure, but the whole passage can still be practiced that way.)
Music theory can help with the "ornamental" notes. Each of them is a half or whole step below the main note, and the determination is according to the key of that particular measure. The first and third measures, for example, are G# minor chords, and the ornaments are diatonic to the G# (harmonic) minor scale. (In fact, the first three measures are in G# minor, and the final measure in G# major so knowing that scale well is helpful.)
But even without this, one could use the Clement "pairs" approach.