Were they together when you got them??
they might be intimidating each other.
get rid of the calci sand
Play sand from Toys r us at £5.00 for 20kg
Try a wax worm or two or those wigetty grubs.
It could also be nothing and just the time of the year.
Ask how they were behaving before you got them.
(you might get an honest answer)
I have three 4 month year olds and they are all behaving differently.
Dont panic unecessarily. watch their weight.