I do know of a Thunderbolt 2 Dock and KVM (Keyboard Video Mouse) device that will allow you to do just this - IOGEAR Thunderbolt 2 KVM, Docking Station, GTD732
There are two key bullet points that address your specific situation:
- Share your peripherals and Thunderbolt 2 devices between 2 computers
- Supports 4K (UHD) resolution. Supports Dual displays with HDMI and Thunderbolt 2 monitors
Unfortunately, I don't know of any Thunderbolt 1 Dock/KVM's. (The MBP 2012 and TB Display are both TB1)
Thunderbolt 2 is backward compatible with Thunderbolt 1 (the main difference being bandwidth) so in theory, it should work just fine especially since you are connecting just one TB display.
The other major downside is that it's extremely expensive. For the $320 USD for this device, I personally would go through the pain of switching cables or (what I would most likely do) get a display with multiple inputs.