Ian,
Very ambitious routing you have chosen using secondary roads. I would highly suggest that depending on the weather conditions forecasted for your date that you pick the one with the best temperature and least chance or rain and high winds.
I would strongly suggest that you run all of it on an interstate highway, out 1770 miles and back 1770 miles. if possible. Check the route with your MS Streets & Trips or other mapping tool.
Why, because it is much easier to navigate when you get tired(and both of you will), is best for gas/potty breaks/fastsfood at approximately 225miles(about what you can expect 2 up on your FJR without fuel cell) emergeny assistance(if needed between 2200-0600 hours). Also, you only need to get date/time stamped tickets at each gas stop and and the start and stop which makes keeping records less complicated.
As Kristie has the desire to do it and you have the strong motivation and desire to do it, then you will be able to complete it within the 24 hour limit.
I am offering this as my experience based on doing the BBG and the 100 CCC Insanity and a failed attempt at doing the BabyButt 1500 miler on a combination of .5 secondary roads and .5 interstates(I was certified as a 1kSS at about 1400 miles as I knew I could not make the full 1500). Max use of interstates improves your chances of safety and successful completion, IMHO.
Good luck. If you need to call me, do so. If you want to use my bike with the fuel cell, also let me know. I do not think using another bike will disqualify you if you are the only one who is the rider. Good luck.