you have to go to project properties:
and then you can enter a command in Post-build event or Pre-build event
for copying files you can use xcopy
command, here is an example(the commands should work without problem, I tried it):
mkdir "$(TargetDir)\assets\MyLibraryProject1" >nul
mkdir "$(TargetDir)\assets\MyLibraryProject2" >nul
xcopy /d /e /y "$(SolutionDir)MyLibraryProject1\assets\*" "$(TargetDir)\assets\MyLibraryProject1" >nul
if errorlevel 1 exit 1
xcopy /d /e /y "$(SolutionDir)MyLibraryProject2\assets\*" "$(TargetDir)\assets\MyLibraryProject2" >nul
if errorlevel 1 exit 1
hope this helps you.