Here is some documentation.
http://support.apple.com/kb/HT5219?viewlocale=en_US&locale=en_US#dispnum
Specifically this, noting the footnotes. I've done some editing for readability:
How many Apple Thunderbolt Displays can I use with my Mac in OS X?
The table below indicates how many Thunderbolt displays can be connected to Thunderbolt-capable Mac computers in OS X.
Computer Maximum number of connected displays
Thunderbolt-capable Macs with only Intel HD Graphics 3000 integrated graphics can support one connected Apple Thunderbolt Display (27-inch):
MacBook Air (Mid 2011),
MacBook Pro (13-inch, Early 2011) and (13-inch, Late 2011),
Mac mini (Mid 2011), 2.3 GHz,
Mac mini with Lion Server (Mid 2011)
Thunderbolt-capable Macs that can support up to two connected Apple Thunderbolt Displays.
MacBook Pro (Retina, Mid 2012),
MacBook Pro (13-inch, Mid 2012),
MacBook Pro (15-inch and 17-inch, Early 2011) and later,
MacBook Air (Mid 2012),
iMac (Mid 2011),
Mac mini (Mid 2011), 2.5 GHz,
Note: The F8 key does not work when using Windows with a USB keyboard connected to an Apple Thunderbolt Display (27-inch).
1 You can connect a second Apple Thunderbolt Display (27-inch) to a MacBook Pro (13-inch, Early 2011) and (13-inch, Late 2011), but the built-in display on the MacBook Pro will go dark. This is expected behavior.
2 iMac (27-inch, Mid 2011) with two Thunderbolt ports supports a total of two Thunderbolt displays regardless of which Thunderbolt port each display is connected to.
3 Mac mini with AMD graphics can support an HDMI compatible device on its HDMI port when using two Thunderbolt displays.
4 MacBook Pro (Retina, Mid 2012) can support an HDMI-compatible device on its HDMI port while also using two Thunderbolt displays.
And here's an OWC article with an image showing a MacBook Pro with Retina that has three attached displays plus the built-in.
http://blog.macsales.com/14241-macbook-pro-15-with-retina-display-can-run-3-external-displays
Best Answer
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:
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.