Be sure that the other brush is also a displacement, and when you want a smooth displacement, select both displacements (ctrl + leftclick) and then there won't apear any holes! When you don't like the displacement, click destroy, to delete the displacement. This makes the displacement a normal brush again. Sewing only works with
displacements who are next to each other, share the same "size"..
this is an example of a "bad" displacement, that can't be sewn:
this is an example of a "good" displacement, that can be sewn: