I did not read all of the replies. It is quite possible that everyone's answers may have fixed your problem. I'm adding my $0.02 to help explain WHY a portable install is a winner for what you _might_ be doing (I say _might_ because all I really know about your intent was that you wanted two installs to stay in sync...).
The other posters who have said that you should do a portable installation are very correct _IF_ you are not expecting to be able to play songs on more than one computer at a time.
What a portable installation does is allow me to install MediaMonkey on an external drive containing all of my music files. I can take that drive containing music+MM and plug it into _ANY_ computer and run MediaMonkey.
One caveat: the drive, when mounted on another computer needs to have the same drive letter as when MM got installed. A necessary-to-have skill is changing a drive letter. In my case it was E: when I installed MM on my new Yoga 920. When I plug it into my desktop machine I sometimes have to change the letter.
What this allows me to do is store my 140K track library on a 4TB pocket-sized hard drive and work on my music (I DJ so I am always crate digging...) on _ANY_ Windows box.
This will get you around the need to sync between installations.
Another thing you will get is simplified backup of your library. All I have to do is plug the drive into a machine and then simply copy the MM install folder and the MM music folder (I have all of my music under a single folder on the drive) to another drive somewhere. (I am dreading that first backup operation... syncing from my drive to the backup drive will be the way to go after that first one...)
There are a number of other folks who say that they prefer other media players (Foobar 2000, for example). I'm probably going to look into doing a portable install of one on that drive so that I can have everything just as I want it without having to keep two copies of the player up to date as well.
Get answers about using the current release of MediaMonkey for Windows.
Thanks everyone for your help, i would prefer not to have to carry an external drive with me, but that is an option. I am going to try to the serial number sync when I get home so the laptop and desktop and media center all have the same music. I'll let you know how it works.