I had not seen the little subtlety!
It works! Thank you very much!
I put the code for people who would have the same problem:
'meta_query' => array(
'relation' => 'AND',
'date_clause' => array(
'key' => 'event_date',
'compare' => '=',
),
'time_clause' => array(
'key' => 'event_time',
'compare' => '=',
),
),
'orderby' => array(
'date_clause' => 'ASC',
'time_clause' => 'ASC',
),