I renovated a Harrisville last summer and had a devil of a time with the shafts, getting them the right height, etc. Finally, I took a page from the countermarch folks and made a set of shaft hangers so that I could hang the shafts from the castle, get them at the right height, and then run the chords. It was much easier both to set up and to run the new chords - get them at the right height. I would think that it would be useful for moving the loom also.
To determine the right height, I just hung a set of weighted chords over the back and breast beams, through the shafts. Using those, I could see how high to hang things so that the warp would go straight through the heddles.
I'm sure that is the reason, and I've seen pictures of wide tapestry looms with two sets of treddles, but these treddles seem to be all in the center in the picture.