Ah, fun! Putting everything on your backs may be a bit ambitious, but it'll certainly streamline your luggage footprint. You will, however, need to decide whether you want to check your bags or carry them with you when you fly. Carrying-on will require smaller luggage.
For you and your wife, I’d suggest fairly large packs to allow room for any common gear (maybe a toiletries kit), or any souvenirs you purchase. If you’re okay with checking your bags, then look into the Osprey Meridian Wheeled Convertible Pack ($329), a nicely designed piece with 4,600 cubic inches of volume. Features include a main compartment with an organizer pocket; a removable daypack; and the ability to switch between a pack and a wheeled suitcase. That last feature might come in handy.
Your wife might look at Eagle Creek's Rincon Vita 75L ($275). It has just as much capacity as the Meridian, but it’s designed for a woman with shoulder straps and a hip belt that are contoured for better fit. And those straps also zip into their own pockets, making it easy to carry like a duffel. The Rincon Vita also has a removable daypack and several handy organizer pockets.
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