Bevo - That's interesting. The story from Paul Bevilaqua is that the SDLF evolved from tandem-fan ideas (not unlike the way that the Harrier evolved from the Wibault Gyroptere) at Lockheed Skunk Works. At the point where I became cognizant of the whole remote-drive fan idea (1991ish) it did already seem that Macs and GE were teamed on the GDLF.
However, it's also quite possible that such teaming was directed from on high.
Engines: I don't know that either system (LPLC or SDLF) is inherently more efficient than the other one. It depends on the BPR of the lift engine (which appears to be lost in the mists of time as far as the GEA-FXL design goes). Of course lower BPR=hotter, but it also means lower diameter and a more slender front end. Then again, it allows the back end to run cooler (mixed flow).