Yes, it is their relationship. Just as any relationship that a child might have with another child is "their" relationship. As parents we intervene in children's relationships all the time -- to resolve disputes, to encourage cooperation, etc., etc. To fetishize the twin relationship as being untouchable is, in my opinion, wrong. Its our responsibility as adults to guide our children towards relationships that are healthy no matter who they are with.
I also don't think its wrong to acknowledge that twinship has certain upsides and it has certain downsides. An upside may be an age mate with whom you have a close relationship who is always there to be played with. A downside may be that there is not a need or motivation to reach out to others where there is a comfortable and convenient person always there already.
Note: I am not saying that there is necessarily anything wrong -- as I mention, it is something I would want to keep a close eye on and if I were homeschooling I would be investing some thought as to how to give my twins opportunities for independent experiences and opportunities to be places and with people where they are not viewed as part of a unit.
"And no offense, Buzzbuzz, but I don't think it's very applicable to compare 4 year old twins who, as a product of their age, don't like to share toys, to adult twins whose relationship does not meet your standard of satisfaction."
I was explaining why this is for me more of a hot button issue. And per my discussions with one of them, their relationship does not meet HER standard of satisfaction but she does not seem to be able to break away from it.