If your tiled backround image is 1920x1080, then you are using it wrong.
Also, two instances of 1920x1080 sprite will take the same amount of memory (a lot!) as 1920x1080 tiled background repeated two times.
Tiledbackground is used to fill large areas of the screen with small repeating pattern. For example - if you need to make a sky, create a small 10x10 px blue tiledbackground and stretch it to the entire layout. Then add small sprites with clouds. This setup will use very little system resources.
Read this toturial: