Hello Regnalf,
Thanks for reaching out. 
Using some custom CSS you can have background that expands multiple Pro Theme Header builder Bar’s. Please add following CSS under Pro > Theme Options > CSS:
.x-masthead {
background-image: url(ADD-IMAGE-URL);
}
Replace ADD-IMAGE-URL with the path of image.
However, please note that for above CSS to work you will have to make the individual Bar’s background color as Transparent. Here’s a screencast that you can take a look.

1- I have found the proper CSS code selector using the Chrome browser Developer Toolbar: https://www.youtube.com/watch?v=wcFnnxfA70g
2- For the CSS code itself, I suggest that you get started with this tutorial: https://www.youtube.com/watch?v=yfoY53QXEnI
Thanks.