This is my understanding of what is going on in this fantastic piece. The sectino you're talking about starts I think in Measure 18. The rigth hand's sextuplets are to be grouped in 3 groups of 2. The point is, that the octaves, which fall on the even count within the sextuplet, syncopate with the left hand's truplet eighths. It makes a fluttering, almost Schumann-esque kind of texture.
Once in a while the left hand has duplets, but the right hand should continue in the same rhythm. So we are hearing not just triplet VS. duplet, but the syncopations of a triplet VS. duplet. It's an interesting cross rhythm.
However that is not the hardest part. At measure 28, the LH goes into straight 16ths. THe right hand, with some different slurring, is still in sextuplets. It' smy understanding that the RH is -still- playing the sextuplets in 3 gruops of two, so we still hear the syncopations of a triplet, now against four notes!! It's really hard, and would be easy, if Rachmaninoff had written the sextruplets in two gruops of three. it would then be easy to coordinate the two hands. It would even be easier if we only had to think three against four, not the syncopated three against four.
Now in measure 30 this is my theory; the RH continues in the same way, that we hear syncopateted triplets. But the LH is playing sextuplets, in 2 groups of 3 each. So the udnerlying rhythm is 3 (RH) on 2 (LH). But the RH should sound syncopated, and the LH not. It's fantastically hard.
I could be wrong about this, but I don't think so. I don't think the Rh in this passage ever goes to 2 groups of 3 in the sextuplet, but I think the LH does.
Definitely to get an idea of what this should sound like, practice hand separate. Or you could practice with the LH playing only its triplet eights, omitting the duplets, and the RH only playing the syncopated octaves. Tghere are a lot of ways to practice it and you should do all of them, because there are so many notes and it is hard to learn.
Walter Ramsey