I know next to nothing about this, but the latest Windows developer's blog for MIDI 2.0 states:
"Now, every MIDI 1.0 port and MIDI 2.0 endpoint is multi-client, regardless of the driver or API used. In most cases, vendor-specific MIDI drivers are no longer needed or recommended, although they will still work if they are kernel streaming drivers.
Multi-client is available for all MIDI 1.0 and MIDI 2.0 apps and devices."
SO it SOUNDS LIKE multi-client is now baked into the Windows operating system.
Or not.