The manual seems to accept specific words (pretty reasonable – an SME is useful, perhaps you?) as:
Corrections/changes/additions may be submitted as a pull request at Github: GitHub - kees-z/DuplicatiDocs: Duplicati User's Manual
I didn’t choose LVM, and on some distros it seems to best be set at initial OS install. This is a big catch, however there’s probably a limited amount Duplicati can do, especially since it probably varies. Manual:
On Linux this uses Logical Volume Management (LVM) and requires root privileges.
and possibly quite a bit more, such as having the foresight (how?) to have installed the right way, or the technical skills to fix things later. There also might be some expectations of the script that looks at LVM:
duplicati/Duplicati/Library/Snapshots/lvm-scripts/find-volume.sh
Invalid Handle to Path was an unsuccessful attempt to debug that with whatever elementary OS set up. Currently (as you can see there) I can’t lead people through LVM. The balance for the manual would be something more than the quote above which ends at root privileges., and less than a full tutorial…
Backing further out of diversions from original questions, It looks like design can handle multiple source volumes on both Linux and Windows. The particular storage allocation used is OS-specific. Linux uses lvcreate --snapshot here to allocate from the free physical extent pool (says its man page). Windows is
How is the diff area location determined?
so it looks like one has a limited amount of control (perhaps using tools such as vssadmin), but not full.