combining two rows wit the Query Builder

You can't do a GROUP BY on an INSERT statement.

You'd need to merge the arrays before you insert them (or SELECT them, then merge the arrays then remove the rows and reinsert the merged array).

Probably something like this.

PHP Code:

// ... after selecting $row1 and $row2
$data = array_merge(array_filter($row1), array_filter($row2));
$this->db->delete('substantially_all_render_temp', [
    'BC' => $row1['BC'],
    'taxyear' => $row1['taxyear']
]);
$this->db->insert('substantially_all_render_temp', $data);