Doing LON with monorails in femurs will have about the same result to mechanical axis as doing internals only as nail prevents misalignment that monorails causes.
But I think it is a very barbaric to have anything external in femurs.
Although I want to do LL in femurs and my budget is not plenty, I wouldn't consider this solution as the scarring, the pain (femurs have much more muscles than tibias and pins cut through them when you lengthen) and the great discomfort don't worth the money you save from doing internal only.
So if you have a strict budget, do external tibias. External in femurs, even LON, is not a wise choice.