I've noticed that in Mail.app for example, if you show the connection doctor, or status or even the address list (Window -> Address panel) while in full screen you can happily drag those across to the secondary/empty screen and they will stay there for the duration of the app being full screened.

I don't think a special API is required, just knowledge of the second screen and then placing windows on that second or third screen.

Parallels also works on multiple monitors in fullscreen mode so it can't be a hidden API.

But still, you can't have one app fullscreen on one monitor and another app in the other.