You need to create two separate slabs. If the outline of your slab is intricate and you would prefer not to retrace one part, you should be able to edit the slab boundary and cut the boundary lines from the area for the "other" slab, then draw in the closing edge lines and accept the edits. Then start a new slab and paste the cut lines to start the boundary sketch of the "other" slab.
David Koch
AutoCAD Architecture and Revit User
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