I see your:

    (
        polars_df1
        .join(polars_df2, on=['state', 'county', 'timestamp'], suffix='_r')
        .with_column(
           ( pl.col('val') + pl.col('val_r')).alias('val')
        )
        .select(['state', 'county', 'timestamp', 'val'])
    )

and raise you:

    pandas_df1 + pandas_df2