Sharepoint - Document Library - show a list of different one

As per your requirement you have to use Content Query WebPart. Its easy to implement as you are new to SharePoint.

You can find more detail regarding using Content Query WebPart to show data from multiple list here Display data from multiple lists with the Content Query Web Part

There is another approach also which uses SharePoint Designer. You can create DataView using SharePoint Designer. You can find short detail here Two SharePoint lists in DataView - Linked Sources with SharePoint Designer

I would suggest using Content Search webpart since it's easier, the processes are below:

1. Insert a content search webpart, edit and change query
2. Since you want to get documents, choose "only return items" and hit "Add keyword filter"

3. Add those two libraries' ID that you can find it in the URL when you click the library setting. Manually type in ListID={xxxxxxxxx}

In the result preview section, you will see the items rolling up from both libraries.