Who's doing the pooping???

Have both a small dog (Havanese-14#) and a cat (8-9#), both are in
apparently good health. One of them has been pooping in the house.
The amount of the poop seems to be more than the cat usually makes in
her litter box, but I really don't know for sure. What is perhaps most
interesting and perhaps the best clue is that after stepping into the
poop (inadvertently, of course), I note that it is rather sticky and
appears to have grains of sand in it. Neither animal eats sand, but I
do believe that the dog may at times eat the cat's poop from the litter
box. Any thoughts? Thanks,
Adam