I would check with your insurance company, but I doubt they will pay for 2 therapists of the same type at the same time. I'd bring your child to the original therapist and send a certified letter to your ex that if she does not agree with your actions, she can take it to court. If she insists on bringing the child to another therapist, insurance won't cover it, and since you don't consent, she can pay 100% of it out of pocket.
I doubt she would win in court. It's already been shown the original therapist has been successful in treating the child.
“There are those that look at things the way they are, and ask why? I dream of things that never were, and ask why not?”