Can I have multiple nested forms within the same fieldset? I am building a custom module that adds a form to certain node types that allows the user to fill/save some additional information about the node.

I would like to have 2 buttons. 1 which saves the data entered (currently working) and another which posts the data to an external script.

I can get this working by creating two different forms and settings their #action's accordingly but I would like the form markup all in a single fieldset, is this possible?
You can have both buttons in the same form. Inside the form's #submit handler (or each button's separate #submit handler), based on which button has been used to submit the form, you can either save the data, or do a drupal_http_request() to execute the external script.

Your form would include buttons roughly like this:

'save' =&gt; array(
  '#type' =&gt; 'submit',
  '#value' =&gt; t('Save'),
  '#submit' =&gt; array('save_function_name'),
),
'process' =&gt; array(
  '#type' =&gt; 'submit,
  '#value' =&gt; t('Process'),
  '#submit' =&gt; array('process_function_name'),
)


Then, the two functions save_function_name() and process_function_name() would do the job.