One is enough and maybe to much
Don't mean to stir the pot but we have just done the Alaska trip this past summer, including the Dempster highway (killer ride).
I, like you was really concerned about carrying extra fuel especially with the two bike. In the end the worry and the effort was all for not. If you have one rotopax you have got enough. If there are roads then there are people and if there are people then there is fuel. I also found that the 800's fuel economy really shines off the pavement where the speeds are lower.
When the road gets tough as it did for us during extreme weather on the Dempster you will wish you didn't have all that extra weight onboard.