[geek1011]

If you have multiple EPUB3-style series entries in the OPF, it should be trivial to modify my NickelSeries mod to put the second one in the subtitle.

Here are the relevant parts:

Spoiler:

NickelSeries@a06f1ac/metadata.h:13-25

    // series is a map of the element ID to its series name and index. The
    // series index is returned as-is and may not be a number, but both the name
    // and index will not be empty.
    //
    // Sources:
    // - Calibre: meta[name="calibre:series"][content]          source =  ns_metadata_t::calibre
    //                                                          series =  [content]
    //            meta[name="calibre:series_index"][content]    index  =  [content]
    // - EPUB3:   meta[property="belongs-to-collection"][id]    source =  [id]
    //                                                          series =  text
    //            meta[property="group-position"][refines=#id]  index  =  text
    //            meta[property="collection-type"][refines=#id] text   == "series"  || none
    QMap<QString, QPair<QString, QString>> series;

NickelSeries@a06f1ac/nickelseries.cc:60-127

static void ns_update_series(Volume *v, QString const& filename) {
    nh_log("hook: updating metadata for '%s'", qPrintable(filename));


    NSMetadata meta(filename);
    if (!meta.error().isNull()) {
        nh_log("... error: '%s', ignoring", qPrintable(meta.error()));
        return;
    }


    if (!meta.series.isEmpty()) {
        QString series;
        QString index;


        for (QString id : meta.series.keys()) {
            series = meta.series[id].first;
            index  = meta.series[id].second;
            nh_log("... found metadata: id='%s' series='%s' index='%s'", qPrintable(id), qPrintable(series), qPrintable(index));
        }


        if (meta.series.contains(NSMetadata::calibre)) {
            series = meta.series[NSMetadata::calibre].first;
            index = meta.series[NSMetadata::calibre].second;
        }


        nh_log("... using ('%s', %s)", qPrintable(series), qPrintable(index));


        bool ok;
        double d = QVariant(index).toDouble(&ok);
        if (ok) {
            nh_log("... simplified series index '%s' to '%s'", qPrintable(index), qPrintable(QString::number(d)));
            index = d
                ? QString::number(d)
                : QString();
        }


        nh_log("... Volume::setSeriesName('%s')", qPrintable(series));
        Volume__setSeriesName(v, series);


        nh_log("... Volume::setSeriesNumber('%s')", qPrintable(index));
        Volume__setSeriesNumber(v, index);


        if (Volume__setSeriesNumberFloat) {
            nh_log("... Volume::setSeriesNumberFloat('%s')", qPrintable(index));
            Volume__setSeriesNumberFloat(v, index);
        }


        if (Volume__setSeriesId) {
            nh_log("... Volume::setSeriesId('%s')", qPrintable(series));
            Volume__setSeriesId(v, series); // matches the Calibre Kobo plugin's behaviour for compatibility
        }
    }


    if (!meta.subtitle.isEmpty()) {
        QString subtitle;
        for (QString id : meta.subtitle.keys()) {
            subtitle = meta.subtitle[id];
            nh_log("... found metadata: id='%s' subtitle='%s'", qPrintable(id), qPrintable(subtitle));
        }


        if (meta.subtitle.contains(NSMetadata::calibre))
            subtitle = meta.subtitle[NSMetadata::calibre];


        nh_log("... using '%s'", qPrintable(subtitle));


        nh_log("... Volume::setSubtitle('%s')", qPrintable(subtitle));
        Volume__setSubtitle(v, subtitle);
    }
}