I don't know why FDNY built a new firehouse for Engine 220 (old Brooklyn Fire Department Engine 20) next to their old BFD firehouse and located Ladder 122 in the old 1883 BFD firehouse instead of building a new combined firehouse for both companies back in 1907.
From earlier posts by G-Man and others:
"For a period of time between 1908 and 1925, NYC built firehouses with walls between companies, and therefore separate housewatches. GMan compiled a good summary a while back:"
"If you go back in time, most ladder companies had their own firehouses including many that had a solid wall between the engine & truck side and separate housewatch desks:"
Brooklyn has a unique pair at E220/L122. E220 was organized on Nov. 1, 1882 at 532-11th St. where they remained until Jan. 1, 1907. That's when they moved into a new firehouse nextdoor at 530-11th St. On that same date, L122 was organized at E220's former firehouse at 532-11th St.
As to why they built separate but adjoining firehouses at that time? Apparently they would assign a captain to each house so if you put the companies in separate houses, each company would have a captain.