I've never seen this before in my life but it's hilarious. One will get up to go get water etc. and the other will scrape her eggs under her. Then the first will come back and get her eggs back.
The smooth feathered one is actually the daughter of the pure silkie one. She's never mothered before. The cochin bantam rooster has always co-brooded the chicks when they pop out too and it's always worked out fantastically.
A lot depends upon the temperament of the hens involved. If they start fighting over the eggs, it can result in cracked eggs and low % hatches. Others co brood effectively. When setting valuable eggs, I never let hens co brood.