A single 40" 4K display is basically 2x2 20" 1080p displays with no bezel space between. That means you would be turning your head quite a bit more with 2x 27" than a single 40" (although with some additional vertical movements I guess). I sit at a distance of about 800mm (~2.5ft) from the display and find I hardly have to move my head at all - I focus most of my work in the top two thirds with lower priority 'peripheral vision' stuff (IM, terminal windows, etc) down in the lower section.