You can do infinite scrolling using just a single tiled background object; instead of moving the X-Y position of the background there's an imageXoffset and imageYoffset value that you can adjust.
Every tick:
-> TiledBackground: Set image Y offset to ((Self.ImageHeight+Self.ImageOffsetY+scrollSpeed*dt)%Self.ImageHeight
You'll find the set offset actions in the "Image" section of the actions menu
This modulo "%" will automatically reposition the offset once it exceeds the image width.
A negative scrollSpeed value will reverse the direction of movement (this is also why you add the image height at the start - modulo can go a bit weird with negative numbers.)
Switch height to width and Y for X for horizontal movement.