Huh? I don't own the book and I know you're pretty much the expert on this, but I'm quite sure I remember reading that you couldn't shape two Soulmelds that were occupying the same slot at the same time, even if neither of them are Bound to the chakra corresponding with that slot.
Now, if either of the soulmelds can be bound to anything other than Soul, of course there's no problem. And unless you're binding them to Soul, you can still use a magic item that takes the slot corresponding to Soul. But ... yeah, I thought unbound soulmelds still had this one slot restriction on the space they occupy.