FlexASIO - I used to experiment with two instances of Gig Performer and it worked, but it was not stable enough for serious use (prone to crashing, initialization errors, text based config (although 3rd party GUI tool exists)).
VoiceMeeter - in my tests it had latency issues, but I see that people use it e.g., this YouTube video for Zoom sessions, and another example is Rocksmith streaming on Twitch (click here to learn more).
I tried Voicemeeter when I was using a Steinberg UR22 audio interface. I can confirm that it worked correctly and without bugs, but I didn’t pay attention to the impact of latency as it was not a live use. The buffer was set to 44.1/256.
This helped me a lot before I bought a Babyface Pro interface.
It’s designed to be an open source cross platform low latency application to application audio routing program.
Some time ago i was going to toss out the idea of the devs looking at the Jack interface (which I believe is available in Juce) as a mechanism for inter-instance audio routing. But i thought i should do some more playing with it before making such a suggestion, and i never got around to it.
ASIO is not the main goal because it’s a Linux/Mac/windows thing, and ASIO is purely a windows thing. But it does offer ASIO on windows.