You cannot create any surface in Civil3d with two elevations at the same x,y coordinate. You would have to create two surfaces for this.
Please use the Accept as Solution or Kudo buttons when appropriate
You can create two surfaces in one corridor and separate them using boundaries. You need to use feature lines to define the boundary.
Once the boundary has been created you will need to adjust the station range.
A loop road with an upper and lower corridor? I'm having a tough time picturing that.
In a loop corridor, you should always try to start and end in the middle of a straight segment. And the start and end elevations have to be the same. Like using a corridor for pond grading.