As Country Bumpkin suggested, it would be better to run MC as a server on the Desktop machine and then connect to it using the HTPC. Is the Desktop PC always-on?
If so, enable these:
Options -> Startup -> Windows Startup -> Media Server
Options -> Media Network -> Use Media Network
Options -> Media Network -> Authentication and set a password.
Then, connect the HTPC's copy of MC to the server running on your desktop. Then, wham, they both run the same Library in MC all the time. Make sure this option is enabled on the HTPC side (it is by default, I believe):
Options -> Media Network -> Client Options -> Automatically Sync Changes
If the Desktop PC isn't always-on, I don't know enough about WHS to advise. If you remote in via VNC or Windows Remote Desktop, can you get a regular Windows desktop and Start button and everything? If so, then you could probably run MC as a server there, and connect both the HTPC and the Desktop to it. But there are some types of Library changes you can only make using the "real, server" copy of MC, so you'd need more info in this case (you can tag items normally from client copies of MC, but you can't change the Library views themselves, or anything like that).
If not, then you could put the Library Files themselves on the network share, and connect both machines directly to them. The way this works is that the first copy of MC that "gets there" gets read/write access to the Library files. Any subsequent copies can connect and use MC, but they are read-only (the way it works is that it lets you make changes, but they are "lost" when you exit that copy of MC). I did this for a long time with my HTPC and my desktops (before they made some needed improvements to the Media Server system a few major versions back). It works.
It is a little annoying because whatever copy "gets there first" always wins, and you need to close that copy to get it to relinquish its read/write "lock" on the library. So, if someone else in your household opens up MC and is watching a movie, and you want to go to the Desktop to do some tagging, you'll need to interrupt them and close MC for a second so that you can open the copy up on the Desktop (after which, they can then re-open MC, say OK to the "read-only" warning popup, and continue their movie). So, using a Media Server is better if you can swing it. But, if you want to be able to shut that Desktop PC down, and the WHS can't run the server, then this might be the next-best choice.