I shared a bank account with Abby before getting married. Having intertwined finances is actually good practive. It's also a dead simple way to split things down the middle. Get a join account, each put in $X/mo., and don't worry about having to split anything after that. Just pay with the joint account. If someone makes Y times more, just put in Y*$X, if you're into punishing success :)
In your plan, do you each have individual accounts in addition to the shared account?
We sort of go the opposite route, with individual accounts for discretionary spending, and a separate account for paying the fixed costs, including the allowances to discretionary spending. We found that spending from a shared account required excessive coordination. It was easier for us to separate the responsibilities and manage our own budgets, like departments in a corporation.
