plug two towers into the same monitor ?

All LCD monitors have three input types - hdmi, dvi and dsub. Is it "OKAY" to plug two towers into the same monitor with different inputs like hdmi and dvi ?
You have to switch the display on the monitor to change the inputs ? will this damage the monitor ?

Yes, it works nicely. I keep a VGA lead permanently plugged into one of my monitors for when I want to test another computer. Then I can jump from my PC to the one under test with just the monitor controls, as you suggest. It works perfectly :)

And how do you connect the kbd and mouse ;)

If you have multiple computers and want to use just one keyboard, monitor, and mouse, then a KVM switch is the answer.

I keep a spare KB and mouse. But if you want to hook something up on a more permanent basis, then a KVM switch would probably make more sense.

Your best bet is to use a KVM (keyboard/video/mouse) switch. Once connected, you just use the Scroll Lock button to switch between monitor inputs. I use them a lot when working on multiple machines at the same time, it's a real help.