How to cut thin strips of Hardiebacker
After scoring, place the sheet on the floor as usual, then cover the thin strip with a stiff board or straight stud as long as the score line. Align one edge of the board with the score. Kneel, or have someone stand, on the board. Pull up on the larger piece. 

In other words, score and snap as usual, but trap the thin piece between your work surface and a stiff strong board.