Apple Thunderbolt Display switching between two computers without manually disconnecting the cord?
What you describe asks for a KVM switch. That would be a small box with 2-4 output ports (non-enterprise class) where you connect one monitor/TB/USB devices and as many computers as you have ports on the switch. Then you switch which one is active and uses screen.

What it effectively does is actually disconnecting computers from the screen, but you don't have to pull the cable, just press a correct button.

A nice feature with those is that you can also connect Windows/Linux PCs and be able to seamlessly switch between all of them.

For example: http://www.atlona.com/Atlona-2x1-Mini-DisplayPort-KVM-Switch.html (it allows to connect 2 mini DP to one screen and switch between them using buttons on the front) [I'm not associated with manufacturer nor the online store, just giving it as an example].