Hi Sam,
Right, yeah, I would try what andrew said, and split them up, as it can cause stress although, I have kept males and females in together for a couple of years with no prolem. Also, have you checked them for worms? Did you say they are not excreting, or they are?
Another thing I would NOT reccomened is calci sand. The females can eat the sand to increase calcium levels, and can cause impaction, so if I was you, I would use play sand, that is a much safer option.
Cheers,
Ian