Hi i'm making a 2D Sonic game that will be about when he was a child and it will be a blaze of speed!! But I heard that there was a second Unity engine where you could make full 2D games does anyone know what it is?
Uh... as far as I know, there is no "second Unity engine" at all, it's just Unity. That's it.
If you want to make 2D games using Unity, you can. Just change your main camera to be an Orthographic camera, and lock your objects onto 2 axes (basically, always make sure position.Z is always 0, or something like that). I've made plenty of 2D games with Unity before, and they all work great.
Mika Mobile go into a bit of detail on how they made OMG! Pirates and Zombiville which are 2 of the best 2D games on the iPhone.
Using a camera with a very small FOV placed really far away gives you almost orthographic view but with a bit of parallax with the back ground / foreground elements.
The characters are done with animated planes and texture swapping. It's clever stuff.
Well, I am not sure what you want to know, but the basic component of a 2-D game is to have a configurable joint on all the objects that can move. (First select the object you want to apply it to, go to Component->Physics->Configurable Joint. It adds a component with a whole bunch of settings. The default settings will have the Motion and Rotation set to free. Change the ZMotion to "Locked". Change the XRotation and YRotation to "Locked" also, you can lock the ZRotation also if you would like to. If you create a player, you need to do the same thing.)
If you need to know any more specific information, just make a new question about the specific topic on what you are trying to do.
Here is a picture,