Couldn't figure a way (I was happy with) to get an aux fuel cell on my 13 R Glide Ultra and retain the tour pack. I switched the tour pack mount from the permanent type to the quick release type. Not a small or in-expensive project. Requires changing to a two piece seat, changing the rear fender mounting bracket covers, installing quick release mount kit. I then bought one of the tour pack mounts that is used
to put the tour pack forward for a single rider and made some brackets to attach a basic e-bay 4.5 gallon square alum fuel cell to it.
Wanted to keep the extra weight forward, lost the capacity of the tour pack for hauling all that crap a guy doesn't need anyhow. Works
great for a single rider. Ended up adding a small rack extension out back to accept a bag for all that crap it turns out I think I really do actually need
Obviously this is a no go solution if riding two up....
On my Multistrada I purchased a smaller 3 gallon fuel cell and mounted it inside a Givi 55ltr rear box. Neither project was easy or cheap!
The plumbing it into the tanks , on both bikes , was a fairly major show as well.