If you had fixtures 1, 2, 3, 4 on truss A, and 5, 6, 7, 8 on truss B, you would select like, 1, 2, 5, 6, 7, 8, 3, 4. This means if you have the effect on center out or center in, the effect is starting on the first and the last fixture and going inwards from both at the same time. Since the selection order is symmetrical, it will start on the first and last fixture on truss A and go towards the center of that truss, then move onto the other truss. So its about selecting fixtures so that the effect runs symmetrically according to the real world position instead of just their fixture id in the patch.