You have to make a severeal TiledBackgrounds with the height and double width of your Layout/Viewport. If you make a endless runner I assume your Layout is similar to your Viewport.
Now you have to set all the TiledBackgrounds half outside of the Layout, left or right depends if you want to scroll to left or right. Now move the TiledBackgrounds with different speeds and set the TiledBackgrounds back to start position after they scroll half of there width.
I made an example, I think its much easier to understand.
Here how it looks like (hope that works)
Sorry for bad english.