I’ve always done multi-monitors in a horizontal set-up. I’ve never seen a good case for stacking them vertically until one of our customers recently showed us an application where it made sense. This customer has two work-flow dashboards which scroll massively left to right. Here we have two LG 29WK600-W 29″ UltraWide 21:9 IPS Monitors running 2560 x 1080 on an Ergotron DS100 Vertical Dual-Monitor Stand. You’d have a sore neck from looking left to right and back if you did this horizontally.
The key to making this work is the stand. It tilts the monitors slightly and allows for the bottom monitor to nearly touch the desk.