From a revenue perspective it's a great idea, who doesn't want a higher AOS? Customers have the ability to shop across brands in a single transaction, good for them, good for revenue. From a brand perspective, however, there is some degradation that will happen (mixing Old Navy and Athleta customers/transaction together arguably dilutes brand equity). As long as the UX is fairly consistent from site to site there shouldn't be an issue, but that assumption would need to be tested and proven out. Ultimately, the customers will decide if it's a good move and vote with their cash.