Yellow bellied sliders

Question
Hi me and my boyfriend went out and brought a yellow bellied slider about 3 months ago, he settled in fast and seems really happy but we thought he could do with some company so about 2 weeks ago we brought another yellow bellied slider, she seemed fine at the time but a couple of days ago her legs seemed puffy and grey we fed them and she didn't eat a thing! Today when my boyfriend woke up he rang me and told me that she is tucked up in her shell and just floating when he got her out she didn't move, my bf keeps convincing himself that she is just sleeping or tired ... could our poor yellow belly be on her last legs?? what could we do to help her no matter the cost?? PLEASE HELP :(

Answer
I'd take her to a vet ASAP.  She may have an infection.  Take her and wrap her loosely in a wet towel and place her away from the other turtle in a quiet, dark spot tonight.  

I wouldn't wait, though... I'd take her to a 24 hour animal hospital and then to an exotic vet in the morning.