Hi @Luulaalaa_A
I would approach this by making the pants a 3D object with some thickness. If your pants are low poly enough you could extrude a thickness to them. Otherwise I would use the Subtool> Extract function with nothing masked to create a new 3D mesh volume from the 2d mesh. You’ll want to do this in such a manner that the interior garment surface intersects and pushes into the surface of the body.
From there you can sculpt only on the exterior surface of the garment by activating Brush> Automasking> Backface Masking OR by simply hiding the interior surface with the mesh visibility shortcuts. If you create a mesh with Subtool> Extract, the interior and exterior surfaces should be separate polygroups which will let you hide them with a single click.
This way you will always have a portion of the garment intersecting with the body and it will not separate. After you are done sculpting this will allow you easily fuse the garment to the legs if that is the goal, or delete the interior geometry if you want to keep it as separate 2D geometry.
