using a second computer as a mere screen/monitor in X (VNC?)
That's what DMX: Distributed Multi-head X is designed to do, unfortunately it's been broken in several X.Org releases, so you may have to work a bit to get it working.
Looking for an answer for myself (using an iPad instead of a computer), I found the best option to be using VNC, specifically tightvnc. See here: https://help.ubuntu.com/community/VNC/Servers#tightvncserver