Please help leo gecko prob

Question
Please please can you help me.
I bought my two son's leo gecko's from local pet shop 7 weeks ago.
I was informed they were only 4 months old and both girls.
The pet shop informed me how best to keep them and i have looked on web sites read books ect for more info.
To start with i kept them together in the tank but it became very clear that one of them was eating all the food and dominating the other, so i bought whole new set up so's to keep them separate as this little one had now lost alot of her weight.
when i first moved them apart she did start eating after a day in her new tank but then she still wasn't putting on any weight and is even now having problems shedding her skin.
i took her to the vets who have given me food to force feed her as she has now stopped eating altogether.
I am confused with all the husbandry that i have read, some say that heat mats are all you need and shouldn't have over lamps as the lights stress them out, some say sand some don't, i am trying so hard to get this right but i really don't no what do she is getting worse and i am so heart broken for her and my son that i could have failed her, the gecko that is.
Ok i have 45 x45 x 60 tank, i have the back of tank under mat heated with a moist pot with live culter which i spray every day to keep moist, this is were the under mat heater is, i am using sand which pet shop have said is safe ? now not so sure, i have also got small wood chip in tank to provide dif surroundings, this is in the cool area, i have over lamp canopy for daylight heat in day, ok the temps are, sand temp with heater mat is 35, air temp with out lamp on just heat mat is 25, with the lamp on it is 30.
i don't put any extra heating on at night as the heat mat's keep the tank at 25.
I have been force feeding leo for a week now and she's still not looking good, very pale, still very skinny and is now even having problems shedding her skin, she doesn't hide any more and just lays on the sand were it's heated not looking good at all.
when i take her out to feed her she does like to climb up and sit on my shoulder. i have hot water bottle with old pillow case to keep her warm while i feed her which she now accepts very well. after i've feed her i've been spraying her with warm water to help with her skin which didn't come off on her last shed but has now completely dryed out, she has a large water dish to sit in if she wanted which she doesn't i also have wax worms in dish with some cal powder, she has eaten one on her own this week.
i really don't know what else i can do for her, if sand is dangerous then i will sort this out as soon as i'm able to afford but what can i use in the mean time that doesn't cost to much.
I am a first time owner and would like to do the best i can if you could please advise me what i am doing wrong and how best to change inorder to not loose her or for the same thing not happen to the other one, who is doing great very big bright colors. no it's not boy i have checked and checked again.
hope you can help us.

Answer
I wish I could help more, but I don't know much about leopard geckos. A few things that might help though:

- Even if they are separate, if they can SEE eachother, it could cause the same dominance and stress issues. This is the case with bearded dragons and some other lizards, and it may also translate to yours.

- The enclosure sounds like it's too big. Especially if they're young, it might just be having a difficult time finding the warmth, moisture, and food. Try a smaller enclosure, at least for awhile.

- Limit interaction and don't move things around in the cage. This also causes stress. The best thing to do would be just make sure there is food and the enclosure is set up right, and give it privacy to get used to the surroundings, and eventually act normal.

- Sand is ok, if it's the right kind of sand. For now, it might be good to get something that you know is safe, like non-adhesive shelf liner from home depot or the like.

I hope it helps, good luck.