FT's were designed by EMC/EMD to come in A/B sets with a drawbar between the two units - it was considered one two-section engine, kinda the diesel equivalent of a Mallet. Originally, there wasn't even a door that you could close between the two units, just the walkway and diaphragms. So FT's were run A-B or A-B+B-A. Later separate FT B units were made without the rear overhang (FTSB's - FT short B's) that could have drawbars connecting them A-B-A.
Santa Fe IIRC wanted to be able to separate the B units and worked out a way to add couplers, other railroads also modified them, I can't recall if GM/EMD offered it as an option or not...by that time they were already working to replace it with the F2 whose were designed to be able to take either a coupler or drawbar.