2 mother hens?

Has anyone ever seen/heard of 2 hens co-parenting a clutch of chicks? I've got 2 that were broody around the same time, they both had their own eggs to sit on. When they all hatched the hens appeared to mix them and now they spend all day every day both watching the chicks. At night they take turns sitting on them. Its odd and I've never seen chickens do this.