How to detect same user when they submit two webforms? User A submit a newsletter form and come to thank you page. At the thank you page there is another general comment form. 

Here is my question: How to detect both of them are belong to User A?

Reason: So that I can extract both information together from database.
Another less obvious solution if using the rules module already is https://www.drupal.org/project/rules_session_vars

Set the variable when the first form is submitted and check for it when the second is submitted. The use rules to do whatever is needed.

Your own prepopulate solution is perfectly fine however. This one just stops someone from hacking the url.