Welcome to the HUBB
As the guys have mentioned, unless you are both physically too tall/big to fit on a 650/800cc, then taking a 1200 is total overkill. Yes riding across Europe on a 1200 will be nice, but as soon as you leave Europe, your average speed drops drastically and try to ride deep loose gravel, slick mud or sandy tracks on any bike with 2 people and luggage, it becomes a different ball game. Don't believe the advertising hype of these big "Adventure" bikes
A few factors though, what is your time frame and route- are you planning on riding dirt pistes or sticking to asphalt?
I would try out as many bikes as possible with your partner and pick the smallest, lightest bike you can get away with - making 100% sure that your partner is comfortable on the back of the bike.
We rode South America 2up on a DR650 and we met a Czech couple 2up on a 350 Enfield in Bolivia, people have done longer 2up trips on much smaller bikes.
As for new/used, I prefer used bikes as they are cheaper and you can walk away from them if they break, get written off or stolen, without losing a big chunk of cash. A good way to look at it is: don't take anything that you cant afford to lose. Don't forget about Carnet costs with a new expensive bike also.
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