leopard gecko (sunshine)

Question
We (my daughter & I...mostly me since she prefers the crested) have acquired a crested and a leopard gecko.  They share a 40 gallon tall aquarium with an artificial grass, a half coconut shell, a stone watering dish, a false stone 'cave', and get daily spritzing and feeding of crickets, we have given her waxworms but not within the last couple of weeks.  She talks to me and bites anyone else.  I am concerned because she seems to be loosing her toes!  I really like her, and feel that she may be suffering, so what can I do to make her more comfortable and stop her from loosing, or eating (I think) her own feet?  They were in sand for about a week and I realized they were having serious problems from walking to clumping and eating it.  Which I understand can cause problems...could this be a problem?  Help me & Sunshine please.

Answer
Hello Mozan,

Well, how much larger is the crested gecko than the leopard gecko?
It is not recommended to mix species & house them together, as their personalities are different & they have very different husbandry requirements, also.  They really do not mix very well.  Can you separate them?  
Do you leave crickets in with them overnight?  If so, the crickets could be biting & chewing them.
It is highly possible that the crested is biting or chewing the toes off.  
What are the temps in the tank?  
Good you changed out the sand to artificial grass now.  
We need to figure out how she is losing her toes & feet.  
Does she have any retained shed perhaps?  
Do you have an undertank heater & a low wattage basking light also?

Tracie