Working while waiting for pending PR

I'm assuming you want to start the new user_story_2 branch on top of the work you've done in user_story_1. Here's the workflow I use in this sort of scenario:

1. Open Pull Request for user_story_1:

     * (user_story_1)
     *
    /
   * (master)
   *
   *
   

2. Create new branch user_story_2 based on user_story_1:

   $ git checkout -b user_story_2 user_story_1

     * (user_story_1, user_story_2)
     *
    /
   * (master)
   *
   *
   

3. Work on the new branch:

     * (user_story_2)
     *      
     * (user_story_1)
     *
    /
   * (master)
   *
   *
   

4. Pull Request gets merged:

     * (user_story_2)
     *      
   * | (master)
   |\|
   | * (user_story_1)
   | *
   |/
   *
   *
   *
   

5. Delete old branch:

     * (user_story_2)
     *      
   * | (master)
   |\|
   | *
   | *
   |/
   *
   *
   *
   

6. Rebase new branch onto master:

     * (user_story_2)
     *      
    /
   * (master)
   |\
   | *
   | *
   |/
   *
   *
   *
   

My preferred workflow for this:

1. On branch master, git checkout -b user_story_1.
2. Make changes to user_story_1.
3. Open PR for user_story_1.
4. On branch user_story_1, git checkout -b user_story_2.
5. Make changes to user_story_2.
6. Once user_story_1 gets merged into master, switch to user_story_2 and do git rebase -i master.
7. This should show you a list of commits on user_story_2 that you want to include in the rebase. Delete the top few commits that came from user_story_1.
8. The rebase should complete cleanly unless master was updated with other changes. Now you have user_story_2 rebased on master and only having its commits.