What can I add that hasn't been said already - and that you didn't already know to start with? Mostly I'm writing this to express my support and send you some warm fuzzies. (And I hope that that isn't some code term for sexual practices I know nothing about, because I didn't mean it that way!)
Recap (with comments):
1) She's a "very nice woman. This is all very above-board and open and the three of us have been getting along great for six months now. It's a fairly equal relationship and both me and the other woman are primaries to him." (How "very nice"? How "fairly equal"? And if she's "very nice", is that your calm, rational, understanding appraisal of her? Or do you actively like her? Does she actively like you? Are you friends independently of the man-in-the-middle?)
2) You can't insist on moving in. You can't insist that he doesn't. And even asking energetically might put you "in the wrong". (Please notice speech marks! Does not signify my opinion but their possible reaction.)
3) She might be trying to manoeuvre you into secondary status (especially if you haven't got that friendship I asked about in 1). But don't assume so. You haven't given us enough facts to judge this. (Not a criticism: we wouldn't be able to judge fairly without actually knowing all 3 of you.) I - for example - would be happy for a very good friend (who smokes) to live next door, eat together every meal, etc. etc. But I don't allow anybody to smoke in my house. Even the best of friends. Even a hypothetical smoking lover. And I would be doubtful of a smoker (even a best friend) swearing that (s)he could move in and live there without ever lighting up indoors.
4) However, even if she isn't trying to downgrade you, the proposed situation definitely puts you at a disadvantage, and frankly I don't hold out much hope for a continuation of a "fairly equal relationship and both me and the other woman are primaries to him." I do think - as you do - that this needs to be addressed, and he certainly should be told clearly that if he wants to continue having 2 primary relationships, he's putting that in jeopardy by moving in with her with you on the outside. After that, it's his decision.
But you already knew all that, didn't you (except for the warm fuzzies)?
Best of luck. Be fair but be clear.
If I can't dance, I want no part in your Revolution.
- Emma Goldman Anarchist and Polyamorous par excellenceThe person who says something is impossible should not interrupt the person who is doing it.
- old Chinese proverbAnd the day came when the risk to remain tight in a bud was more painful than the risk it took to blossom.
~ Anais NinI'd rather have a broken heart / Than have a heart of stone.