Southern Route

I plan to ride across the country on an electric tandem with my wife when I retire next year. We have had a tandem for more than 30 years but not an electric tandem. We plan to go the southern route starting in San Diego at the end of February. This is the bike I am considering.

https://mobilityscootrike.com/product/fat-tire-tandem-electric-bike-2-people-person-bicycle-dual-2-two-seat-e-bike/

I am concerned with the battery distance, about 30 to 40 miles. We would like to ride more than than each day. Has anyone used two batteries, and advice on charging? All feedback and advice is welcome.

Thanks