That is what I would do as well. “Oh, this must be [baby’s name]. He/she is beautiful! I am so sorry to hear about the loss of his/her sister/brother.”
Outside of maybe sending a sympathy card, I don’t understand why you would bring up the dead twin ever. If they bring it up, certainly be supportive, but it just seems like it would be weird to start talking about in any manner on your own.
I’m not religious and I would not appreciate any sort of statements along these lines. The only thing that would come close is if they said, “[Lost twin’s name] will always be a part of you” because that is scientifically true and very heartfelt.
Rand Rover, while I can acknowledge that having twins is a huge, expensive pain and it would be easier if there was just one, I’m at the point now where I have two babies (rather than blobs) and the loss of one would be just as devastating as the loss of a single stillbirth. I’d prefer to have the child acknowledged, and I would greatly appreciate those who did, but YMMV.
Many responses seem to assume that the live birth is the primary one to be acknowledged. I suspect that for the time being at least, the loss is weighing very heavily indeed on these parents. The live birth, after all, is the common, expected event.
I would suggest being very careful to avoid saying anything that suggests the live birth is more important, or that it in any way mitigates the loss. While it would of course be macabre to focus on the dead child in the presence of the living one, I would otherwise suggest, contrary to some advice above, that the sense of condolence for the loss be paramount, exactly as if their born child had died shockingly just a couple weeks ago.
I have had great “success” in situations of grief (death, cancer diagnosis, loss of a child, etc) by simply walking up to the person and giving a sincere hug. They know whats going on, they know you know, both of you know there are no words that will help. The hug shows support without creating any need for conversation…which is always awkward and or trite.
Come in, hug, let go and carry on. Message sent.