Should I get a side-by-side or a stacked (over-under) dual arm?

Side-by-side is the default for two equal-size monitors used together. Stacked or over-under mounts, like the Ergotron LX Vertical Stacking arm, suit narrow desks or users who want one monitor as a secondary display above their primary. Reviewers note stacked configurations need a tall pole and adequate clearance behind the desk.