[McAroni]

I could write in German, but for the better understanding for all others I will try it in English first  . 
Try the following:
Select your songs you would like to put in another library
Click share and export as msf and save the file.
Create a new library and name it as you wish 
Activate the libraray
Import the saved msf file. 
Now the songs are in the new library. 
To synchronize the library's you have to create a library with the same name on your second device and activate it. Then you could synchronize your library.
As I know only the activated library will be synchronized. 

Try it first with a small amount of songs and see if this works.