For 2-up, make sure you get a big enough top case that can easily fit 2 helmets, that way you can lock them up if you stop on a trip.
Donít worry about one size looking ďout of proportionĒ too big. The difference between a ~40L case and ~52L case is only an inch or two in each direction. Once you put a topcase on a bike, it already changes the look so significantly, a slightly larger one really isnít going to look so much more out of place, so you might as well go big.
In regards to brands, Iíve only used Givi, but make sure you get the Monokey, not Monolock. Monolock is the lower-quality line designed for scooters, Monokey ones are a little more rugged.
Donít worry about needing a backrest pad on the top case. They tend to push the passenger more forward, and since your passengers jacket likely has a back pad/armor, extra padding isnít necessary.