If there's anything anywhere that shows #9 green as a so-called double green I'd certainly like to see it to try to determine what the reason for it might have originaly been.
I've looked at some early drawings of the course, and I've looked at the enormous mock-up of the course hanging down in the main maintenance building I think for the purpose of determining if #9 green was ever intended to be much bigger or "double".
If it was ever intended to be double when the course was first designed it seems logical to assume that it may have been that way or planned that way simply because the old inn was behind #9 green and #9 was originally #18 and #10 was orginally #1. If that were so it would seem logical that the present green would have been the back of a double green and probably would have served as a practice putting green/18th green much like the well known #9 green/practice putting green melded combination of Oakmont.
But if the #9 green's double section extended well into the present fairway approach, with basically that front section serving as the primary green area of #9 I certainly can't really see how that would make the hole or its second half more interesting or more challenging. In my opinion, it would very likely make 9th hole and its second half less challenging and probably more one dimensional.
Furthermore, it seem very unlikely that any double green section would've been behind the present green section. All one has to do is go behind the present green to determine that. Of course, it would be possible to build another section back there but that would seem to be more invasive of the architecture of NGLA that just putting a really well conceived narrowish bunker scheme in a diagonal of perhaps 90 yards in length extending across the second half fairway connecting at the fairway bunker right out about 150 yards and ending at the bunker on the left out about 60 or less yards before the present green.