Port of the week is now presenting you x2x which stands for X to X connection. This is a really tiny tool in one executable file that let you move your mouse and use your keyboard on another X server than yours. It’s like the other tool synergy but easier to use and open-source (I think synergy isn’t open source anymore).
If you want to use the computer on your left, just use the following command (x2x must be installed on it and ssh available)
$ ssh -CX the_host_address "x2x -west -to :0.0"
and then you can move your cursor to the left of your screen and you will see that you can use your cursor or type with the keyboard on your other computer ! I am using it to manage a wall of screen made of raspberry Pi first generation. I used to connect to it with VNC but it was very very slow.