I am wondering if this is a limitation on Windows, I am using win 8.1.

Yeah, that sounds like a reasonable explanation.  On Windows only one app can open up a MIDI port at a time.  Someone posted on the forums a few weeks ago that this is no longer true in Windows 10; with Windows 10 multiple apps can connect to a MIDI port…  I haven't had a chance to try that out myself though.  So, I can't verify that that's true…