It IS possible to run multiple instances of NetVSTHost on different ports. The problem (my fault) is that the GUI doesn't display which IP/port a given instance is using, so when you have several open at once, it's hard to keep track of which is which.
The last IP/port you set gets saved to the Registry as a convenience, but if you open a second NetVSTHost, its attempt to open the same port will fail, so you can just go ahead and set a different port number.
If you go back to the first NetVSTHost instance and select Devices->Network, it will show the updated values from the Registry, but the program won't attempt to use them unless you click OK. If you click Cancel, it keeps using whatever it was using before.
I'm sorry this is so confusing. I had a look at the code today, to see if I could add the current IP/port to the NetVSTHost window title (to help you keep track of which instance is which), but I didn't succeed. That code is old, very complex, and wasn't written by me. (Which is why I have never published it.)
Until I have time to create a newer alternative NetVST host program (or figure out how to display IP/port in the existing one), you will need to find your own ways to keep track of the settings for multiple NetVSTHost instances. For example, I suggest you create them one at a time, assign them increasing port numbers, e.g. 27016, 27017, etc., and lay out the windows in a corresponding order, so port numbers increase from left to right or top to bottom.