Taylor Swift and Justin Bieber, seperately they have the ability to ignite hysterical shrills from young teenage girls. Imagine what happens when they're together on the same stage!

According to our sister-site, PopCrush.com, the Bieb made a surprise appearance during Taylor's show in Los Angeles last night. Throughout the duration of her current Speak Now tour, Swift has worked cover songs into her set, including an acousitic rendition of Eminem's Lose Yourself during a stop in Detroit a few weeks ago.

It almost seems as if the two have done this before as they trade verses back and forth flawlessly, with Bieber showing off his rap skills by handling the verse originally performed by Ludacris. In my opinion, pre-planned or not, this is a smart move by Bieber seeing that his girlfriend, Selena Gomez, happens to be Taylor Swift's BFF and as we all know it's always in the boyfriend's best interest to keep the girlfriend's friends happy because when the friends are happy, the girlfriend is happy.

Check out last night's performance in the video below along with Taylor's Eminem cover from a few weeks back.