Hi,
Have you considered separating them . . . . . . so to reduce the possibility of competition for food.
You could try D. Hydei fruit flies ( commonly used ....largest species) for the little guy ?
Leave the little guy in his own little enclosure for a while to make dinner 'easy'..... yummy !
The need hiding places ( holes ) -- way up high in their enclosures.

ie----silicone a 'reptile' cave to the side of the tank wall -- leave drain holes- they would love it!