I agree with Kyz.  I have a mixed flock for eggs to satisfy my liking for diversity.  Then I have purebred Columbian Wyandotte, purebred Buckeye  and purebred Cochins as well as Bantam breeds of Delaware, Spangled OEGB, Rosecomb and D'Anvers.  I also have project pens for Blue Columbian Wyandotte, Blue Birchen and Black Birchen Wyandotte.  A lot depends on what space you have and what your budget can afford.
Diana, if they are still fighting over the nest, then yes I'd separate them and give Olivia Australorp the majority of the eggs.  Have her cage all made up with food water nest material and the eggs right in the nest.  Then gently add her to the cage.  She should settle down nicely when she sees those desired eggs she wants to hatch.  The game could probably handle two or three if you want to let her.  Otherwise Olivia should be able to cover them all.  
I gave an OEGB hen six bantam eggs to hatch and then gave her more chicks.  She is biddy raising 15 Bantam chicks and doing a great job.  She looked really determined trying to cover all those babies under her wings in the cage for that first week.  Now she is on the floor with her brood and beginning to take them outside.