title: Multiple json_object_agg functions in subquery return syntax error
body: <p>I have two tables</p>
  
  <pre><code>CREATE TABLE service (
      id SERIAL PRIMARY KEY,
      image VARCHAR
  );
  
  CREATE TABLE service_description (
      id SERIAL PRIMARY KEY,
      service_id INTEGER,
      language_id INTEGER,
      title VARCHAR,
      subtitle VARCHAR,
      col_left TEXT,
      col_right TEXT
  );
  </code></pre>
  
  <p>I'm trying to retrieve the last 4 columns in the second table as objects with language_id as key.</p>
  
  <p>The following query works (ommited col_left/right to save space)</p>
  
  <pre><code>SELECT
   s.*,
   (
     SELECT
       json_object_agg(language_id, title) AS title
     FROM
       service_description sd
   ),
   (
     SELECT
       json_object_agg(language_id, subtitle) AS subtitle
     FROM
       service_description sd
   )
  
  FROM
      service s
  </code></pre>
  
  <p>Though when I do </p>
  
  <pre><code>SELECT
   s.*,
   (
     SELECT
       json_object_agg(language_id, title) AS title,
       json_object_agg(language_id, subtitle) AS subtitle
     FROM
       service_description sd
   )
  
  FROM
      service s
  </code></pre>
  
  <p>I get err code 42601 - syntax error. What's wrong?</p>