I have the same question ("Is it possible to display both the full image AND the zoomed in view concurrently to check for focus").
I cull my images for both focus and basic composition (for instance, if any part of the subject is cut out of the image).
The only partial workaround I have found is displaying on a monitor the main view with the snapshots at the largest setting, and on another monitor the close up view ("preview") at 100% or 300%.