Create a document with the top and bottom items. Use this document as background.
Add a first background, then add a second one.
Did you try?
Where is the issue?
When i add the 2nd background (only at bottom), the 1st background (only at top) goes away and vice versa.
In this case I guess that the 2nd background is not transparent and cover the whole page.
So you need to crop it, or to replace its white background by a transparent one.
It is only possible to add one background on a page.
Oops! Sorry, I use a French version, so I made a confusion with Watermark.
You can add several watermarks, and you can use a watermark as a background when using the "Appear behind page" option.