There is no simple (automatic) way to track 2 users (of a single library).
You can Tag: OnHis, OnHers
when connected is easiest:
ondevice: true; Select all; Bulk metadata edit and add (the) Tag
Removal is similar:
ondevice: false; Select all; Bulk metadata edit and REMOVE (the )Tag
Percentage is a whole nuther thing. If you want Numeric sorting, the custom columns FOR EACH user. Then it is MANUAL ENTRY
(you might be able to semi-custom (by device ID) this in Kobo Utilities Extended, since it does allow per device rules)