After fooling around on my PSR-E443, here are a few suggestions:
(1) Turn off DUAL and go through each of the Organ voices until you find one that's closest to the basic sound you're looking for. Skip any voices that turn on the DUAL option, since having two sounds layered together will just confuse the issue. I felt that the Pipe Organ voice sounded closest to what I was wanting, but you might prefer a different voice.
(2) Now turn on the DUAL option, then hold down the DUAL button until the Dual Voice comes up in the Function menu. Set the Dual Voice to the same voice you just chose for the Main Voice.
(3) Go backward in the Function menu to adjust the Main Octave setting, then go forward to adjust the Dual Octave setting, until you get a result you like. For instance, I found that setting the Main Octave to -2 and the Dual Octave to 0 gave a nice "full" sound that I liked.
(4) Now adjust the Main Volume and Dual Volume settings to get a balance between the two voices. You'll probably want to set them to at least 100, or possibly higher. Also, you might want to set them both to the same value.
(5) I think a large part of what gives a cathedral organ its rich sound is the amount of reverb in the cathedral, so adjust the Main Reverb and Dual Reverb to get a richer sound. I tried cranking them both up to 127, but you might not want to go that high.
(6) Now go forward in the Function menu until you get to the Reverb Type, and adjust it to see which setting you like best. I thought that Hall 3 added the most expansiveness to the sound.
(7) If the results still fall short of what you're looking for, you can try cranking up the Main Chorus and Dual Chorus, and experiment with the Chorus Type.
( 8 ) You can also try layering two different voices together, rather than the same voice at two different octaves.
I hope you can cook up something that satisfies your palate!