Make the contents of two folders appear in one

This can't be easily done with the tools available in Windows.

Libraries are not regular folders but are virtual folders. Not many applications can use Windows Shell objects directly which are not file system objects.

So a script which creates hard links in "all" for all the files in "Assets" and "Images" seems to be the easiest way.

cd all
rem Remove all the files in all
del /f /q *.*

rem Create hard links to files from assets and images
for %I in (<full-path-to>\assets\*.*) do mklink /h "%~nxI" "%I"
for %I in (<full-path-to>\images\*.*) do mklink /h "%~nxI" "%I"

Here %~nxI will expand to file name and extension only, without the path. If the folders are on different drives, you have to use symbolic links: remove /h switch from mklink command.

Run the script above every time the contents of "Assets" and/or "Images" is updated, or run it periodically.

A better solution would watch for file updates in "Assets" and "Images" and synchronize the changes to "all".