Another few hours have passed, and I'm still woefully outside my comfort zone. I've googled "song writing duos" and just about every other possibility, but nothing of use appears. Doubtless it would help if Ifor had told us what the nature of the thematic location of the important 20 letters would be, but that penny has yet to drop. A hint as to the identity of the duo would be useful. I'm assuming that the rarlier suggestions that it's 3,4 + 5,8 stating with J are correct. Starting until bedtime appeares my likely fate. The devil in me is contemplating composing a puzzle where the vital clues are hidden in the key signatures of Bruckner symphonies.