Firstly let me Explain what is pinned post?
Admins can pin one post to the top of their group.
Pinned posts remain at the top until they're removed or unpinned.
Pinned Post can seen by any Member of that group and top of their group/Under pinned post section.
Now,Come to the exciting point.
>Firstly pin a post that you need to Pin
>Then "Deactivate" that Facebook account from which you had pinned the post.
>Now,Pin another post that you need to pin.
>Now, "Reactivate" First Facebook account by logging into it.
>Done! You had 2 Pinned post in your group!