While 3D modelling, you might need to separate two solids that have been previously united
The command Cut Solid By Curves is used to cut one or more solids
Make sure that the option Split Solids is ticked
1. Drop the Solids to Surfaces
2. Add any missing Shapes (shapes are missing because you have previously united the solids)
3. Use the command Stitch (within the Modify Surfaces tools)
The command Stitch will allow you to stitch the bounding shapes into two separate solids by selecting the surfaces as needed.
The surfaces and shapes can be stitched with a selection set. If the stitched surfaces create a closed shape the shape will be converted to a SmartSolid.