I have 2 BG sections in my projects with a parallax background. But because they are part of the same CSS, when I change one, the other changes as well. So now I have the same image twice.
The way that you can handle your issue is by simpy adding a combo class to one of the sections and after that changing the image. To add a combo class select element you want to update and on top right of the screen, you will see blue tags with classes, press next to them and type in name of a new class, after that change the picture and it will change it only there rather than on both places.