Then my hunch is probably correct: I think lots of folks try to simultaneously install aircraft into both FSX and P3D and yet it will only work in one or the other... although typically only FSX. That it works in P3D legacy mode seems to essentially confirm that.

I can't blame you for wanting to keep your FSX installation (I've moved on, so I can't entirely relate) but we need someone to chime in that's successfully installed a CS aircraft--or any 3rd-party aircraft--into both simulators at the same time in order to see if there's a method that works.

Not to upset CS (although I'm becoming doubtful that they meander into their own forum with any regularity these days) but I'm pretty sure you don't need to purchase another 777. Once you have a setup with only one simulator, I'll betcha it works.

If you have space left on your hard drive(e.g. 40+ gb), you could try partitioning off a large block and installing windows, P3D, and the aircraft to ensure that it works. If it does, delete the partition and merge it back into the rest of the drive. But that's a lot of hassle...