Dipper and Mabel invited you to the Mystery Shack. So, you walk into the shack, and see that the twins are already waiting for you at the threshold

"Hey! We'll show you something!" Mabel says with a smile

"I hope you like it" Dipper said, nodding his head slightly

You stood there silently, waiting for what they wanted to show you. The twins turned their backs to you, while putting on black glasses for coolness

Dipper had 'mystery' written on his back, and Mabel had 'twins' written on her back

They leaned their shoulders against each other And the inscriptions came together If you read them together, it means 'mystery twins'