
I guess it depends on the dogs.
I have had whelping boxes side by side and the dogs right from the start were cleaning each others pups. One of the mums also kept taking the others babies and putting them in her box! (Not recommended if all the pups are same colour obviously but mine weren't) All the other dogs would visit too and climb in the box but that is not how I have ever done it before and obviously you have to do what your bitches are happy with.
Mine were both first time mums and it was very much if one stepped out the others babysat.
I know other breeders who could not have bitches side by side even when the pups were older as the mums were so protective, depends on your dogs.