u/Maleficent-Tale-5770

In this toccata, according to the score, the alternating chords are often split between swell and positif keyboards (R and P). He asks at the beginning for full swell, 16-4 reeds and mutations, and principals 16-4 on the positif. Initially the swell is coupled to the positif but later on he asks you to play the P uncoupled as the chords are split between keyboards. Why ? The boxes are closed and the P, being only foundations, is much much quieter than the full swell. I can’t figure out the musical reason. Is it something to do with the organ he was working with ?