sync 2 computers auto function

Expecting that you have the same user on both computers (so the paths in the rekordbox library are the same on both computers), I'd try this:
1) place the rekordbox library and your music directories on a Dropbox or Google drive (this will ensure the content and files are the same on both computers)
2) Symlink the original locations of the directories so they are pointing to the Dropbox or GDrive directories (so the REkordbox is accessing the shared content but still thinks its using the local one)

This is pretty easy to do BUT you must always ensure that the files are NEVER accessed from the Rekordbox on both computers at the same time. Also make sure the absolute path to the files remains the same on both computers and you should be fine..
Haven't tried this with RB yet but I am using it with other apps regularly and it works pretty flawless and I see no reason why this could not work with the RB... - the symlink is a magic how to do such things :-)
Make a dummy library you can mess with and give it few tries.

@Marek,

Thanks for your time!

Were could I find the original library? 

I already have all the musical files on dropbox.

@Steven

Just tested and works pretty well for me (good enough that I will keep my library on the GDrive....).

What I did:

1) My original library created by RB, was in "/Users/maara/Library/Pioneer/rekordbox"

2) I copied the directory to the GDrive (all my music is already there, so I am moving only the library, the music remains unchanged. For Dropbox, its the same, but the path will be different.....)

cd /Users/maara/Library/

cp -r Pioneer /Users/maara/Google\ Drive/

So now I have copy of the library on the Google Drive....

3) Now you can remove/rename the original library

cd /Users/maara/Library/Pioneer

mv rekordbox rekordbox_original

4) ...and create link, pointing form the original destination to the GDrive

ln -s /Users/maara/Google\ Drive/Pioneer/rekordbox rekordbox

5) So now it looks like this (there is no directory, but a link)

~/Library/Pioneer ⌚ 20:15:56

$ ls -la

lrwxr-xr-x  1 maara  staff    43B Sep 12 20:15 rekordbox -> /Users/maara/Google Drive/Pioneer/rekordbox

6) If you start the rekordbox, it should now use the "shared library" from the GDrive

Thats it. On the 2nd computer, just do steps 3 and 4 as the library is already on the dropbox or gdrive, so all you need to do is to make the link...

Please note that you should do a backup before trying it - even you will not delete the original library but only rename the directory (and in fact you will have its copy on the dropbox), better safe than sorry and I am not responsible if you screw it somehow :-)))

Also if you have any doubts about the commands (even they are dead simple), rather don't mess with that..

One last thing - this is for OSX. On the windows, its also doable as win7 and above do support the symlinks, but the commands will be different...

.....and don't forget - NEVER EVER let both your computers write to the library at the same time...