THEY TELL ME NOW.
Since they share a litter box, Siren most likely has the same parasite. So they said since he ain't pooping, just bring in Siren's poop and they'll check it. If by some chance there's nothing in her poop, then I'll have to bring his but chances are she'll have the same worms/worm eggs and that's all they need to determine meds.
Of course they tell me this after I threw out her poop. Now I must go into the garbage to obtain said poop.
I'm going to cry.
My life is full of poop.