The black and white one is simple than the Hello Kitty one. What I would do (and this is just me, not sure how anyone else would do it....)
For the black and white: I would first roll out the black, measure the circumference of the cake, and then cut the black in a strip so that I can wrap it around the cake. Yes, there will be a seam. (Cool thing about black if you make it yourself and use chocolate: The seams are pretty easy to blend together.) Then, I would roll out the white and cover the top, with the cake on a turn table so that I can trim the excess. The draping/garland will hid the seams.
For the Hello Kitty Cake: Same process as with black. Once it's on, go back in and with an exacto knife cut the pattern you want. Roll out the white and cover just as in the black and white cake, trim, then use your exacto knife again. This ope will hid your seams on this one.
If you cover in one color then roll out the second color and put on top, there are two things you'll experience: 1)depending o the colors used, and how thin you roll, colors can show through. (Think chocolate or raspberry icing showing through thin rolled white fondant). 2) you'll have more "dimension" on the cake. The second layer of fondant will stick out father than the 1st.
Hope that helps!