Order twig array by date and time
Thanks to brad I got the whole thing working. 
Here's the stripped down code: 

{#

    Get all Dates from within the table field in the section veranstaltungen
    ------------------------------------------------------------------------
    We have a section called veranstaltungen with a table field that contains dates and times
    In order to get all occurences of the dates wihin the table field, we first get all of the
    table dates and attach the entry to each table date.
    Then we sort this array (dates) first by day and within a day by time.

    This is a simple Calendar without the need for a plugin.

#}
{% set entries = craft.entries({
    section: 'veranstaltungen',
    with: [
        'cf_veranstaltungs_cat',
        'cf_veranstaltungsDatum_table',
        'cf_veranstaltung_asset',
        'cf_veranstaltungsLinks_table',
        'cf_veranstaltungs_assets',
        'cf_coVeranstalter_user'
    ],

    relatedTo : relation_param,
    authorId: veranstalter_param|trim,

    limit: null
}) %}

{#
    The dates Array will contain all occurences of the entries - we will order it first by date and then by time
#}
{% set dates = [] %}

{% for entry in entries %}

    {% set table = entry.cf_veranstaltungsDatum_table %}

    {% for row in table if row.startDate %}
        {% set dates = dates|merge(
            [{
                'date': row.startDate|date('Y-m-d'),
                'time': row.startTime|date('G'),
                'startDate': row.startDate,
                'startTime': row.startTime|date('G:i'),
                'endTime': row.endTime|date('G:i'),
                'entry': entry,
                'links': entry.cf_veranstaltungsLinks_table,
                'image': entry.cf_veranstaltung_asset,
                'files': entry.cf_veranstaltungs_assets,
                'categories': entry.cf_veranstaltungs_cat ?? null,
                'categoryColor': entry.cf_veranstaltungs_cat[0].cf_veranstaltungsCatColor ?? null,
                'categoryIcon': entry.cf_veranstaltungs_cat[0].cf_veranstaltungsIcon_asset.first.url ?? null,
                'author': entry.author,
                'coVeranstalter': entry.cf_coVeranstalter_user,
                'id': entry.id,
            }]
        ) %}
    {% endfor %}
{% endfor %}


{#
    ------------------------
    Veranstaltungen Listing
    ------------------------
#}

&lt;div id="list_container"&gt;

    {% set datesSortedByDay = dates|group('date')|supersort('ksort', 'date') %}
    {% for key, day in datesSortedByDay %}

        &lt;h2&gt;{{ key|date('l, d. F') }}&lt;/h2&gt;

        {% for date in day|supersort('sort', 'time') %}
            {% include "veranstaltungen/_common_tableDates" %}
        {% endfor %}
    {% endfor %}
&lt;/div&gt;


Crazy what you can do with twig!