Place the semolina and all-purpose flours and the salt in a food processor or the bowl of a heavy-duty mixer. With the machine running, add just enough water to from a stiff dough. When the dough forms a ball, remove it from the processor. On a lightly floured surface, knead the dough until it is smooth and elastic, about 1 minute.
Line 2 large jelly-roll pans with clean dry dish cloths or napkins. Dust the cloths lightly with flour.
Cut the semolina dough into 8 pieces. Take 1 piece and keep the remaining pieces covered with an inverted bowl as you work. On a lightly floured board or countertop, roll the piece into a long rope about 1/2-inch thick. Cut the rope into 1/2-inch lengths. Hold a dull-bladed knife with a rounded tip in 1 hand with your index finger pressed against the blade of the knife. Flatten each piece of dough, dragging it slightly so that the dough curls around the tip of the knife to form a shell shape.
Place the cavatelli to dry in a single layer on the prepared pans while you continue with the remaining dough.