I'm using round join, see the first image.

What I want is round join to affect the inside curve of a line join as well as the outside curve of a line join.

Round Join:
Click image for larger version.  Name: Screen Shot 2011-09-28 at 13.41.27.png  Views: 242  Size: 9.7 KB  ID: 84338

What I want:
Click image for larger version.  Name: Screen Shot 2011-09-28 at 13.43.11.png  Views: 202  Size: 9.8 KB  ID: 84339