Just use 2 separate supplies (with appropriate current ratings). Or a 12V supply and a buck regulator.
There are power supplies with 2 outputs, but they are generally designed for a specific purpose, eg. PC power supplies which can provide 12V and 5V. I am not aware of any common dual supply that provides 12V and 6V.
There are three ways I can think of to do this off the top of my head.
The first would be to spring load the poster in the retracted position. When the motor loses power, the poster retracts on its own. When the Arduino is powered, it can drive the motor to open the poster and hold it there. Mechanically this is more complex. It requires well balanced forces,...
I would probably go for a belt tensioner.
figure : Belt tensioner (source tecscience)
i.e.: at some point in the assembly install a belt between two pulleys and use a motor to engage the tensioner.
This is a very simple arrangement, that should satisfy your criteria. The only drawback is that it can be a bit bulky.
you can use a toothed belt to ...