You can not, there is no such thing as a three way curve end. See there ate two simple generalizations of curves:
- Each and every segment is separate, this can not span an area
- Or a multi segment curve where each curve end follows a second end. This allows you to make closed areas.
Admittedly you could expand this to a network mesh. But the complexity from a users perspective would blow up. Meshes are just harder to handle and you would have even more cryptic problems than you have now (how would you round a 3 way join?) And quite frankly theres more things we could use our time on.
So how do you solve this? Essentially 2 ways, which incidentally are the related to the ways to model thing above:
- Use a corner type that is the same in all configurations. In other words use round corners. No problems.
- Don't think of the internal lines as closed shapes, but separate lines.