We tried this a couple of years ago and immediately there are no solutions (as far as I know)
We tested this solution First you need a
source of result both for your library and list. Then you create a
query rule that you consult the first source of results, tell your document library, and then in the query rule add a
block of results which passes the query to the second source of results. In the result block dialog box it also gives the block a label that the CSWP can use to collect the results. Finally, on the page, configure the CSWP to select the default value (first part of the query) and another CSWP to select the result block.
After all that, the biggest drawback is that Only 10 items can be sent in a block of results. At least when we tried this several years ago, that was the limit. So ultimately it was useless for us.
Is there any reason why I can not use a single CSWP and mix the elements and documents? You could add a refining element / document.