I got my first bike over year ago and it was a 1200GS, I did learn to ride on a 650GS for a few months but it really felt wrong for me and I am glad I had the opportunity to do that so I could realize a small bike would not suit me or what I wanted to do. I am 198cm and weigh 100kg and wanted to do the long distance tours on road and gravel. I test road every bike under the sun and liked the 1200 the most. Being so tall I had a great advantage over my sister who at around the same time got herself a 800GS (which is taller), but by the same token we have both now done some really interesting riding with both these bikes and managed to get through everything we tried. I have even taken my 1200GS to parts of de Wildt and Maraisburg and managed to get through although quite tiring. I love the fact that a can ride all day on my 1200GS with luggage at 140+ k/hr and still do some rough off roads.
For you I would really suggest a 800GS or the like (maybe KTM 990, Triumph, etc.) It will require some practice at first, riding just one-up but soon you will be confident to ride two-up long distance and do gravel. If you want to do two-up riding I would be sceptical about using a bike less than 800cc especially with luggage. I can't deny it will be easier the learn to ride on a smaller bike but then again what in life is easy. Oh and how ridiculous do you look on a 650, I know I did with my long ass legs.
lastly when you both decide to get your own bikes each then you already have your first really nice bike, then you just need to get a smaller 650 for the other half.
+100000 for good safe gear, GOOD helmet, neck brace, proper boots, pants and jacket.
Not matter what bike you get go ride it and enjoy - and welcome