Obedience training can help too. I'd start with corgi number 1. And I'd put her on the "Nothing In Life Is Free" program. Reinforce that YOU are the alpha her and that YOU make the rules she needs to comply with.
Nothing in Life is Free
http://www.goof.com/~pmurphy/NILIF.html
Nothing in Life is Free Gaining control of your dog humanely
Still I would not allow them together unsupervised by humans. And yes, this is from my experience also. When two corgi girls decide they hate each other it's very hard, if not impossible to change minds.
And also from experience, I have seen where spaying works. I had two sisters that didn't get along until both were spayed at age 7. Now if I had known, I'd have spayed both earlier. So Yes, my first step here would be to spay them. Step two would be obedience and NIFL programs.
And keep in mind that just maybe one of the corgis would do better in a home as an only dog. If they both get along with the lab then it would be up to you to decide which would do better in another home.
Peggy