This lively prelude begins with persistent solid chords that serve as pillars to hold up a line of quick-moving eighth-note figuration. Both parts carry the same forte dynamic marking. Later in the piece it becomes clear that both hands are to play on the same manual, the Great.
Mendelssohn's own preface for the sonatas describes “FORTE as indicative of the full organ without the admixture of any of the fullest stops.” What Mendelssohn does not offer the performer in specific direction he does offer in freedom to customize the registrational plan for the organ at hand. The vagueness of his directions is intentional:
“But in as much as every organ of which I have cognizance required, in this respect, its own particular treatment, owing to the fact that the like-named stops on different instruments do not always produce uniform effects, I have confined myself to prescribing certain limits only, without actually indicating the specific stops to be used.”
He describes the next step, FORTISSIMO, “as suggestive of the full organ.”(Quotes from prefatorial note by the composer to Op. 65.)
If Mendelssohn's forte is just short of full organ, clarity is probably not a high priority. In the manual we could build to chorus mixtures of moderate volume and perhaps an 8' chorus reed, all with balanced (probably coupled) Pedal.