This isn't about money. This is bizarre.
My dad and stepmom keep separate checking accounts, but they pool the majority of their money for household expenses. They determine who contributes what depending on their average income at the time. So if they're making similar incomes, they each pay for half the household expenses. Then they can use the rest for disposable stuff. They each have their own kids, and like to be able to buy gifts for each other, etc. My dad is a broker so his income varies a lot from year-to-year - this keeps it pretty fair. I never hear them complain.