Use two (2) identical make and model screens on your WorkFit-S Dual workstation if possible. That way they will line up perfectly when installed.
If this is not possible and you have mismatched legacy screens there is a workaround. Order the Tall User Kit which will allow you to adjust the height of the screens independently so that you can line up either the top or the bottom of the screens.
As a bonus, the Tall User Kit will also allow you to tilt the screens independently which is a nice added feature!