This is an interesting trend from the US; businesses that operate different ecommerce sites under different brands using tabbed navigation at the top to switch between stores. For example:

I haven’t come across any major UK examples yet. Potential sites might be the Arcadia brands (Topshop, Topman, Dorothy Perkins, Miss Selfridge, Wallis, Evans & Burtons) – all very distinct online shops which don’t even link to one another. At the very least you’d think there would be a synergy for TopShop and TopMan, which are often linked up in the physical world.

Gap/ Banana Republic is another candidate but they don’t have a transactional website yet. Interestingly their US site allows you to shop online for Gap, Banana Republic, Old Navy and two other brands using a universal shopping cart.
There’s a huge difference between the brands (Old Navy is cheap and cheerful, Banana Republic very expensive and Gap in the middle) but Gap Inc. obviously see a benefit for them and for consumers in tying the brands together. Rather than a branding no-no they believe “the best brand strategy is to do things that customers love, not what the brand marketers think customers want”.