Select your single layer design. Create a copy of the part you want to make the 2nd layer. I copied the outer layer of the truck because I want to create a second layer of the bottom half. Center your copy on the original, sometimes it is easier if you make the copy layer a different color.

Select your copy layer and turn on nodes.

Select the extra nodes you won't need.

Delete your extra nodes.

Align your remaining nodes in the order you want them.

For this design I collapsed the top of my truck into the line dividing the cab from the bed.

You will see from my final picture that it created a layer for the bed. This design now has 2 layers.