I do that with two tablets and a laptop. On the notebook I run some vnc server with an X built-in, then some x or vnc tool that links the different X instances running as if they are other monitors, so you only have to move your mouse to some lateral to get focus on the other X. Then on the tablets just use some generic VNC client.
There are multiple options for ever one of the three programs os this setup, but it takes some work to make all run well together. I can check the exact binaries that I'm using or send the scripts, if there's someone interested.
Edit: to make it work well enough with wifi, I had to make some tricks to deal with the tablet's wifi powersave behavior. On usb it works perfectly though.