Beyond Compare 4's Picture Compare has 2 images on top and the composite image below. This suits landscape images, but wastes plenty of space for highly portrait images on 16:9 monitors:
This video and screenshot show it operating in side-by-side layout:
http://youtu.be/AKsbjFX4GN8
and the Picture Viewer plug-in for Beyond Compare 2 has "Side-by-side Layout" and "Over-under Layout" options:
http://scootersoftware.com/download.php?zz=v2pixview
Is there a way to turn on Side-by-side layout in Beyond Compare 4's Picture Compare? Thanks!