Work with her by herself. Think of this as training and not as a walk. Once she learns then you can consider it a walk and take both dogs. You are not there yet!

At first you may need to "stand like a tree" for what will seem like eternity. If you don't get a response in 3 or 4 minutes, turn and start walking in the opposite direction. When the leash gets tight immediately turn again in the direction you were originally going. Try again. Before she has a chance to have a tight lead again, give her the verbal YES and a treat. She will catch on.

It is going to be harder to unteach the bad behavior that she has learned than to learn the new behavior she must learn. In other words, she needs to unlearn that pulling will take her where she wants to go!