1st the 650 will handle 2- up with no problem, even @ 80. That being said, taking 2 bikes will allow you to bring back more stuff you find on your trips.
Back roads, IF you know them, and less traveled roads are good place to familiarize yourself with the bike.
Start with nice weather, then windy weather, a steady 15-40 mph. Head winds and crosswinds can be challenging, and different, at different traveling speeds, on different bikes.
If there will be 2 bikes, ride to the weakest riders ability, so there is no pressure "To Keep Up", and learn the characteristics of both
If riding 2-up, ride so the passenger is comfortable with your riding style.
When comfortable, then the expressway. The expressway later because at higher speeds wind is a factor, especially along side a semi while traveling at 80 Indicated, during a crosswind.
I say indicated because at 80, your closer to 72. The speedos are off about 10%. 60 indicated is 54, etc. Doing 55 indicated in a 55 mph zone, you will be "Leading the Parade", actually doing 50, frustrating those behind you.
When your up to it Rain. When you go on a trip, expect it.
I would start out with a 150-200 mile day trip, (shakedown cruise) to check that your body can handle it. Everyone is built a bit different, and you may find that you need to make some ergonomic changes. (seat, backrest, grips, etc.).
I would not recommend a tight schedule for a bike road trip. Relax and go with the flow, budgeting 150-400 miles a day. (some days are better than others)
As always, "Ride like your a Target, or Invisible".