I have done it many times. Hens that do it are often the same social grouping either as sisters or mother daughter. This appears to involve hens imprinting on each other in much the same way they imprint on chicks and chicks imprint on siblings and mother. The problem that can arise is where mothers either get into physical altercation with other or where they have different ideas about what brood it to do. Hens in such arrangements are fluffed up more than single parent hens indicating to me more stress. When you take into account the number of chicks hatched and reared to weaning (about 5 weeks), on a per hen basis the single hen arrangement will yield more chicks with fewer injuries.
As a result I normally suggest removing one hen from the brooding effort. If two hens to still be used in a free-range setting, then invest more effort in having waters and feeding sites with creep feeders scattered about where hens will be foraging. What you will note is hens co-parenting will differ on when to alternate between feeding / drinking and brooding which can be particular problematic during the first few days. Once chicks are pushing two weeks problems will be largely worked out unless hens differ on when to roost up. When hens separate but where chicks can hear both like might caused by a fence or wall, the chicks can become distressed.
