Greetings.
I think I know what kind of problem you are facing. I am practicing the 5th Moszkowski etude(C major op 72), and on the 3rd page I encountered arpeggios and scales in the right hand and the long arpeggios in the left. At first it was difficult to do both hands simultaneously, but when I learned each hand thoroughly, namely the right, the both hands played more naturally. The similar thing I find in alot of Bach's polophonous music, as in his preludes. My main advice to you is to study each hand very slowly and carefully, paying close attention to flexibility and a good sound.