Well, it’s not possible by default, but can be done in a seamless manner after you install the jailbreak app called Front Cam Un-Mirror. The release is not available in the public repo of Cydia, but this post includes the repo address for you to try.
Front Cam Un-Mirror includes a button at the top left corner of the screen when you open the front facing camera. When you tap the button, the tweak wastes no time in quickly un-mirroring the front facing camera.
The button design may make it difficult for you to register touches; other than that, it works quite well. There are no configurations to make; just add the following repo to Cydia and make the installation for free: http://cydia.myrepospace.com/sticktron/
So what do you think ?