However, if you recall, the panel schematic combines the two lines from each of the OVRD/JETT pumps into just one line, before it then feeds vertically down into the manifold
Oh, I can see how your aircraft panels may be confusing. My company aircraft (most of them retired now) P4 panels depicted two individual vertical lines, one from each centre pump down to the horizontal cross manifold. (no 'Y' junction) I've dug out my old training notes and they are the same. The Boeing panels book that I have though shows the 'Y' junction that you are familiar with.
We had following combinations 747-200 P & W, 747-200 RB211, 747SP RB211 and 747-300 RB211(still a few of these kicking around)
It's been a while since I've seen one of the -200 panels so maybe our old girls did have the 'Y' depiction!!!