how to add multiple tables in hook_schema() in .install file while creating custom module?
You definitely can!
In your hook_schema implementation, your first array key is the table name. You can add as many as tables in this array: 

Ex:

function quotes_schema(){
  $schema = array();
  $schema['quotes'] = array(
    'description' =&gt; 'Base table for Quotes. Contains all submitted data from initial page.',
    'primary key' =&gt; array('qid'),
    'fields' =&gt; array(
      ...
     ), 
  $schema['quotes_access'] = array(
    'description' =&gt; 'Access tokens to quotes.',
    'primary key' =&gt; array('qid'),
    'fields' =&gt; array(
      ...
     ),     

    ...
  return $schema;
}


This will create both quotes and quotes_keys tables.