Ignore the theatre but it appears fundamental differences in taxation policy.
There is an argument I have heard from a couple of economists and people close to the industry that the duopoly, as unpalatable that it is, manages to keep prices down, even though they do line their own pockets along the way.
Reason being is they more or less dictate terms to the food supply chain relentlessly pushing prices (they pay) down. We often hear the pitiful amounts farmers get which often rightfully get some sympathy but also remember there are times that the farmers make bank bigtime at well. This price pressure also follows on to the big food manufacturers who put simply need Woolworths/Coles business (and often both of them) to be viable.
If we broke up the big 2 then the pendulum will swing the other way with the suppliers having the pricing power. Sure retailer margins may go down although I doubt it'd be all that much. I think I read somewhere that WOW net margin went from 5.3% to 6% recently which means although their input prices have gone up, they have also added a bit for themselves at the retail cash registers. The problem is I just can't see how any business aside from banks (they are an entirely different proposition) that that can (or will) even bother trying to do it for less on what will be an arguably higher cost base.
What does spinning them off actually give us? Satisfaction we broke up the duopoly. Yes. Lower margins by the supermarkets? Probably not - same same at best. Lower overall cost? Nearly certainly not.