Is it possible to have two different skies in the same worldspace? for example have the normal sky at south and a sovngarde sky at north with a gradual transition?

when I create a worldspace (a city) having a parent worldspace how do I tell CK where the city's worldspace should belocated in its parent worldspace?

You'd do that with http://www.creationkit.com/Regions#R.

And a child worldspace uses a parents worldspace landscape. So you just do it in the same coordinates.