We can add our experience to this post, we have a trio of Golden Partridge Pekin Bantams and the two females went broody at the same time, but were not sat on any eggs so we put 6 eggs under them as we had quite a few that were due to go in to the incubator.
We did as Badger did put 3 under each, after a week we noticed that they had split it 6 under one and 4 under the other so we changed it back, this happened again when they popped out for some food so we decided to leave them be.
Because the trio are in a separate area with their own little house we put some food and water in little dishes just in case they wanted something, they did occasionally come out over the 21 days but we were not overly concerned.
They eventually hatched 4 of the 6 and are now bringing up their chicks as a happy family of 1 cockerel, 2 Hens (Obviously 2 mummies or a mummy and nanny) It is so sweet when they wander round together with the little chicks jumping on and off the girls backs and onto the little outside perch they have.
This is our experience and we thought we would share it.
Regards
Robert & Sarah.
South West Poultry.