Sometimes it works; sometimes it doesn't. Depends on the mood of the mums at the time. An in my experence, they're just as fierce after a couple of weeks as they are at the beginning - maybe even more so!
Last year I had threee bantam cochins, a mum, daughter and grand-daughter, each with her own clutch of chicks. Since Cochins are generally so gentle, we put them together in a big (60' x 30') run. They were ok for a few days, then started scrapping; we'd separate them, and things would be ok again for a while, then they'd go back at it - none of the three would ever back down. One day I came home to find the oldest had been dreadfully beaten up - her head was all bloody and both eyes were swollen shut. She's made a complete recovery, but it was slow.
Interestingly, I have another hen who seems far fiercer, but she does't have prolonged battles - she confronts fiercely, repels all invaders, and then retreats back to her own family.
Now we have smaller (15' square) runs for each mum, and never keep two mums together.