These aren't like the trucks that are almost crazy-glued to their rails. I find that the center area has lots of contact but the ends are sloppy. A single bearing I can rock end to end but if you install two on one rail and bolt a bar across them they are completely steady in every direction (except around the rails since there's nothing stopping that motion).
One on each bar as shown with something bolted across them would let you rock them back and forth in the directions of the shaft ends. You really need four of these on any axis - two on each rail.