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2 piece longerons are used because 6061 sheet aluminum is only 12 foot long
I thought they were 2 piece on the 701 for shipping purposes although on the 750 I think they were one piece.
from what I understand the 750 uses extruded aluminum for the longerons which would be available in longer lengths, The 701 longerons are formed from .040 flat sheet, In 6061 the full sheet length is commonly 12 feet. They could be built out of 2024 which can be purchased in any length since it is commonly manufactured as continuous roll, but where will you find a brake with the capacity to bend that full length. They could be built from extrusion but that equals more weight.
I've always surmised this was the reason -- in any case, it certainly seems to work. I'd guess this is one of the least modified aspects of the 701...