I'm studying Rachmaninoff's Prelude in C# minor and towards the end, the staff lines are joined together, with two ending lines.
Do I need four hands? :)
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Thankfully, four hands are not necessary. Nor are (both) feet.
The two lower staves are for the left hand, and the two upper staves are for the right hand: from bottom to top
LH LH RH RH. Notice that in most cases, when the lower LH and RH staves have chords, the upper LH and RH staves have rests. Play the left-hand chords, then jump up to play the right-hand ones, then back down for the left-hand ones.
This arrangement is used to indicate two distinct parts: the lower, more ominous chords; and the upper bell-like chords. By splitting the staves, it can help illustrate the distinction. See also When does one use an extra stave for piano music?
In the case where the chords are simultaneous — the first and third in the indicated excerpt — treat the lower staff chords like grace notes slightly before the beat, and play the upper chords on the beat.
This is not a rule, but it is a strong convention. When arpeggiating chords, for example, the assumption is to start from the bottom. Similarly, when playing chords as in this piece, the working assumption is to go from low to high. The low-first convention is evident throughout this particular piece, with no indication in should be changed in these particular measures. As one example of many, here is Evgeny Kissin. The video is timed to the measures in question.
The below edition (Ray Alston, ISMLP) illustrates this explicitly.
For more on this notational convention/technique, see When does one use an extra stave for piano music?