He needs to be separated from the other gecko immediately! Unfortunately with the way he is looking and the fact he is not eating, he may not survive much longer. Right now you have tried all you can to get him to eat and can only continue trying to see if he will eat. With his weight, I strongly advise sending off a fecal sample to be tested for parasites. It could just be stress, but better be safe than sorry. I am not sure if you are using UVB bulbs or not, but using one with him may just help.