When you combine objects with ctrl+J, it creates a single object. It doesn't merge the meshes in the sense it makes one contiguous mesh. You'll either boolean the meshes together (easiest) or manually merge verts into one mesh. Hopefully that makes sense.
I already control J and combined these two meshes but while sculpting they won’t blend together. As you can see there is a crack between the two in the center I can’t get rid of.