Revit includes a Stacked Wall system family for modeling walls that comprise 2 or more subwalls stacked on top of each other. The subwalls can have different wall thicknesses at different heights. All the subwalls in a stacked wall are attached and their geometry is joined.
Only wall types in the Basic Wall system family can function as subwalls. For example, you can have a stacked wall comprising an Exterior Brick on Metal Stud and an Exterior CMU on Metal Stud that are attached and joined.