Problem is, I keep forgetting the magical incantation. So for the following setup:
Desktop, on left
Laptop, on right, IP address 192.168.12.205
I do this:
ssh -X 192.168.12.205 x2x -to :0 -east
There are other options out there for controlling multiple displays with the one keyboard and mouse... for example, Synergy can do more than 2 screens, and works on Windows and Mac as well as Unix. It's quite good, and I've used it in the past, but it needs to be installed, and you have to set up the config files, so it takes a bit more doing.