Just because a set of characters go out on their own, does not make it
part of the old series. (Tagged: Series Spin-off, just like happens on TV or the movies
)
Then there are Series that have Branches (Webers Honorverse) or
Cycles (CJ's Foreigner) Those I use Hierarchies: Honorverse:Saganami
The index is always for the placement in the smaller unit.
Fractional Indexes to keep the place between full units (or to flag the rest of an omnibus: 4.57 Places the start at 4 and has 5,implied 6 and 7)
Other people keep a custom: series field to log the second (parent?) series