I have several computers with monitors attached on my desk. I use only one keyboard and mouse between them, though. This is accomplished by using Synergy and QuickSynergy to share the mouse and keyboard with the PCs.

Simply, the PC with the mouse and keyboard acts as the "server," and the other PCs are "clients." The configuration (which is where QuickSynergy helps) defines screen edges that are used to switch between them; move the mouse "through" an edge of the screen, and it appears on one of the other monitors. The monitor associated with the mouse is the one that also gets the keyboard focus. This works with Linux, Windows, and Mac, and probably others if you try (source is available).

The recent upgrade of my Ubuntu ('cause it's easy and for simple desktop OS, I'm lazy) seemed to break Synergy. I couldn't get the mouse to reliably cross the screen boundaries to the next configured desktop. For my permanent desktops, the client is to the right of the server, so hitting the right edge of the server (or left edge of the client) should cross the boundary. Sometimes this works, but not often. More accurately, it sometimes works moving from server to client, and when that happens, it always works moving from client to server.

When I attach the Mac to the eqation, I have it configured "below" the server, so moving past the bottom should move the mouse through the top of the Mac desktop. Again, it sometimes worked, but always worked on the way back. I configured it to go through the bottom because it got difficult to "pop" open the Dash (formerly Unity) icon bar when it hid. To recover the bar, you had to get so close to the edge to make it pop open that the mouse would swap before the bar would open. Clicking the icon on the menu above the hidden bar also popped it open, but for the main system I found this to be more bother than help. Plus the Mac is physically lower than the main monitor, which suspends about 10-inches over the desktop, so it kind of made physical sense, too.

As a brief aside, for those considering a more complex mapping, if you don't use QuickSynergy to manage the server (as I do), you can inject a more complex configuration allowing portions of the screen to act as gateways to different machines. Also, at least with the Windows server GUI, you can "chain" desktops more, so you could continue left through a client to another client, or what have you.

It's a little boggling because the two desktops are both running the same version of Ubuntu (now Oneiric Ocelot, or v11.10). I could understand more if the behaviour was different between different versions of the OS (more because they're different versions of the Synergy software than anything else).

Today I discovered that the trouble with Oneiric's switching seems to be limited only to the "clear" parts of the desktop. If I drag from the server to either client through one of the menu bars (either the top or icon bar), the mouse always transitions as expected. This makes sense why it always works from the Ubuntu client to the server; that side is where the icon bar is (it's a secondary machine, so the interaction with the bar was less of a concern, even though the edge has the same issue). Also, it made sense why it works always from the Mac back to the server; 'cause it isn't the same desktop.

I don't know what the edge of the desktop is doing when not dragging anything. I say that because if you drag a window to the edge, after a brief pause the workspace will switch and the window will be on a different workspace screen. Perhaps the edge detection of this is getting in the way of the edge detection of Synergy. Regardless, going through the very edge (through the menus) isn't such a bad compromise; in some sense, it actually helps enforce the edges of the screen a little more. It is quirky, though, that sometimes hitting the desktop edge does work; perhaps there's some random chance that Synergy gets the edge detection first.

I'm not sure where to report this behaviour, so maybe someone who does will see this and can report it to someone else. That is, if it's a problem for anyone else. Or if it's a problem for others, perhaps this will help enlighten them to a solution; add menu bars to the edge, or remove the workspaces feature (I haven't tried that, though) or controls.

At least I have a working solution for still using just the one set of keys and mouse.