You can place multiple pictures on your slide (or slide master), but
each slide can only contain one as it's 'Background'. Either place
them directly on the slide, or combine them into a single file in a
photo editing program.
Perhaps you can insert the graphic image 1 on to your slide using the
insert menu (not as a background just the way you would do or a
regular insert image) . Resize, crop and adjust the position and
transaprancy to your liking. then do the same with the second image.
Now export the slide as a jpg or other image format (export png if you
want to retain alpha channels). Now insert this combined image as your
backgound.