Just found this thread. I did have some trouble when I let two broodies share the "broody apartment." One of them, a d'Uccle, hatched out yellow chicks with stripes. The other, a bantam Cochin, hatched out some yellow with stripes and some dark brown chicks. The d'Uccle conceived a hatred for anything that didn't look like her own chicks, and began attacking. She killed two little browns before I figured out what was going on. Since then I've tended to keep broodies in their own little enclosures . . . but even that doesn't always work. Another broody (an EE/Silky cross) with seven chicks of all different hues still went off the deep end and started killing chicks belonging to another hen when they wandered into her pen, even when they looked like her own. I guess they really do know the voices of their own chicks!
Needless to say I don't incubate eggs from either of those hens. Not good genetics.
That said, not all hens go bonkers. The bantam Cochin sisters I have are able to share chicks without anyone getting hurt. And one of my Icelandic hens went broody again when her single chick was a month old. When the new chick hatched, both mother and daughter raised it. It was really funny to see that little month-old girl brooding her baby sister!