Scary stuff, Todd! With luck some of our cat gurus will be signing on later today and maybe will have some advice for you.
Did the vets do bloodwork and a urinalysis? Any chance they got into something that might be attacking their kidneys? That can often result in arched back and weakness in the hindquarters (at least in dogs) from the pain.
I do hope you get some answers soon and your kitties are on the mend quickly!