GameCenter Leaderboar and Achievements at Once...

I don't believe it's possible. Granted, I don't have access to the source code for these methods and classes, but I'm assuming that each time one of these is displayed, it is embedded in a UIView that is created on the fly. I'm pretty sure you won't be able to display both of them at once due to how the code was most likely written on Unity's side.

You'd have to make your own plug-in to do such a thing.

Yeah, I figured it. I was thinking about doing like in joyride jetpack. The leaderboard is exactly the same with the unity code but it has a boton in the left corner that shows you the achievements. I dont have any idea to do this.