have tried cutting the circle in half? (one defence owns the left side and the other defence owns the right).  shooters should always be spread out to keep the circle open so they can drive into space so normally they will split from side to side.  so if you had a drill where your shooters are practising this with a ball and they rotate left to right, one high one low, while your defence picks each of them up as they enter their section.  you can have key words like STICK to mean they stay on their current player, SWITCH meaning switch sides, and the likes.  but having a good defence communication is more about knowing what they are both expecting the other to do, than talking.  so the key to a very strong defence is to have solid positions for the girls.  let me know how you go.