Multi-table form

Hello,

My module creates a new content type and I use Schema API to build the DB tables. My problem is the following : one node creation form must filled two tables instead of one as usual.
So the code :

function my_module_insert($node) {
  drupal_write_record('my_table', $node);
}

is not sufficient.
I need something like :

function my_module_insert($node) {
  drupal_write_record('my_first_table', $node_first_part);
  /* get the id of the row just inserted (am I forced to use a SQL request for that ?) */
  drupal_write_record('my_second_table', $node_second_part);
}

Perhaps I must split this content type in two. But can I use one form to build two nodes ?

I have no idea how can I do that stuff. Is it possible ? What is the best way ?

Regards.