I guess I'm wondering why it matters which broody raises the chicks as long as at least one of them is doing the job? Maybe the communal broody pen with all chicks together would work - I guess it depends on your flock and whether those two pullets get along. The babies will go to whomever takes care of them and make that chicken the one they follow. I think the only way to force the broody to keep their own chicks is to separate them for awhile. Maybe put them in separate pen area for a couple of days with their individual chicks so they imprint somehow. Sorry - hope someone with experience can chime in.