I had success with 2 japanese bantam moms sharing the nest, they shared the chicks and both were mom when they hatched. They did great.
BUT... I had 2 serama hens share the nest and they failed because they laid eggs at different times and kept pulling the eggs under them back and forth. When one from the first batch hatched they both continued to keep tucking the rest of the eggs back and forth and neither of them got up to teach the baby to eat I believe. They covered him and kept him warm but neither hen would leave the eggs behind to help him and after 2 days he died. It upset me and the rest of the eggs went into the bator so I don't know what would have happened, but a few more hatched right away, and a few almost 2 weeks later so I'm sure if I did nothing some more would have been lost.