We had a similar incident at our vet's office. I agree with everyone here who says it's just not worth putting two dogs in a confined space, unattended particularly, without muzzles at least. Greyhounds just can't get away from normal dog-tangles without bloodshed, with their thin skin, but why risk any dog to this kind of stress?
As to who was the aggressor, I'm developing a hypothesis that it's the aggressor who can be the most injured as it's the other dog who's fighting for his/her life after an initial attack, which might not leave the biggest mark. That's how I imagine it went in the case of my two dogs at the vet's. Pogo has been aggressive, and attacked Brilly in the past. I've seen how Brilly will try to avoid attack, but will defend himself impressively if cornered. I'm sure that's what happened in the kennel they were put into together.
Like you, I was scared to death that all the work we'd done, all the progress we'd made towards peace between them would be destroyed, but we haven't had a single grumble since then. We still do all the things we've learned about managing and muzzling and training that have brought us this far, but none of that would have made my dogs safe under those circumstances.
I hope your girls can move past this and heal psychically as well as physically, and that you are also able to let it slide into the past, only taking the lesson with you.