In Preferences/Power Management there is a setting for what to do when the lid
closes. It might be relevant to what you want to do.

I have left the 'when laptop lid is closed: blank screen' and
changed 'put the display to sleep when inactive for' from 30 minutes to 'Never'.
I already had 'set display brightness to: zero and 'Dim display when idle'
checked.

Hopefully this will keep the display active, but dimmed to zero.

I use a KVM switch that allows me to use three computers on one monitor
,keyboard and mouse and it changes with one button of on the keyboard
shortcut. As for the laptop it probably over heating with the lid shut,
I leave mine open. While mine work a treat I do find I sometime lose the
mouse after a swap.

I think KVM switches have circuitry that (more or less) mimics the
mouse, keyboard and monitor to the computers that are not connected to
these devices. I say "more or less" because sometimes the offline
computers aren't fooled and refuse to work when they are switched to
active. They may detect the "fraud" only after the passage of a few
minutes.

While it hasn't happened to me often I have had the same problem from
time to time, especially when the computer is rebooted and then
switched "offline" before it gets fully started. I don't know of any
easy solutions, but maybe someone else does. Perhaps a better quality
KVM switch may help, but some are extremely expensive and I go for
cheap...

Ubuntu is picky with KVM switches, I keep handy an USB mouse to plug in if the
ps2 mouse doesn't work. Sometimes the keyboard goes, too. Luckily my
monitors aren't a problem.

It does not matter which operating system you use the KVM switch only
swaps the signal around. I swap between Unbuntu, XP, Vista and Win 7 and
even plug in computer that I am repairing/ testing. No its a better KVM
I need but in these tight days I watch the pennies. You have a switch
with both PS/2 and USB ports?

No, I use separate mice and keyboards on my push-button or dial KVM switches.
Lately, on a customer's computers she upgraded to a wireless mouse, so I brought
along a USB printer sharing device that I bought for a scanner, but alas it
didn't work for it...I'm glad to report that she can now share 4 computers with
her wi-fi mouse. The 4-port KVM is still using the ps2 keyboard and VGA video
just fine.

At one of our local recyclers, I saw they had some of the bigger KVM switches,
but they used dedicated cables, so I passed them up. I've seen some on
Craigslist, too. Once I bought an 8-port KVM from EBay, but not all 8 would work
on it, so I traded with another tech that's better at soldering than I am.

Mine was bought on eBay, Not the top of the range but works OK and the
mouse problem never really bothers me. The switch circuitry is not the
problem in my view. When I reboot it work at once. Its a Logitect
Trackman thumb ball mouse and the KMV uses PS/2 connections and this I
suspect is part of the problem

Having used KVM's for many years I've found that the definitive
answer, at least for the 'budget' range of KVM's, is it's down to the
motherboard how well ( or bad ) they work with KVM's. I've had systems
that will only boot if the KVM is switched to them and others that
only boot if it isn't. Then there are those that boot differently
whether switched or not, and that can be a whole range of things from
hanging at a different phase of the boot cycle to the OS failing to
start up correctly, or the graphics resolution to be different.

Also found that the quality of the KVM cables makes a huge difference
in boot reliability too, and it's not always the more expensive ones
that work better !! One set I had worked a lot better after wrapping
the entire length with kitchen foil - evidently a lack of screening
here - they aren't used any more

Perhaps a more up market KVM would be less problematic but whilst the
budget range does the job I'm happy to live with their foibles

The Jumble of cables are a major factor especially
because a small unit has anything up to 20 odd leads clinging to it and
these can often work loose, I find I need to boot each computer with the
switch to that one until all is OK then on to the next. Cannot just boot
each computer and hope the switch recognizes them all. I have used them
for years also and have never got one thats foolproof