I did have a weird keys patch where i was jamming with it but going to a normal keyboard sound makes it clearer to hear what im doing instead of letting the reverb disguise the sound. It seems like by your description your instinct would be to go for bass on left and guide tones (3rd and 7th) on right and then alter from there?
that's always cool, if you don't know the song yet and specifically what it needs where. also it's good to practice shuffling the range around so the chord/harmony generally is situated around the middle C range. that's just where all the harmony wants to be. i think the more music you check out, the more you will find that that's almost always where the actual harmony is taking place. you may have many instruments helping out, but there are never actually that many different notes being played. the harmonic content is really usually in the middle C range and slightly above. that's just something that i've found over time. when you hear a dense arrangement with a lot of instruments, i think you'd be amazed to find out that there still aren't that many different notes being played, there will be a lot of instruments playing the same notes (layering)