I can't help with the windows VAC but I've gone into Quisk and played with multiple RX (not something I've done much of) and while you can create more than two RX and monitor multiple bands there are only two distinctly separate audio sinks available and to get audio to the second one it must be set to a digital mode. So, I could get audio from main RX or which ever you select the play button on to the main audio sink and totally distinct audio from a second RX to the Digital RX1 sink if a digital mode was selected for that receiver. These can then be routed where ever I wish in Linux with Pipewire. In windows it would be some sort of VAC app.
So this is essentially the same as running two receivers in piHPSDR except piHPSDR creates a second audio sink and does not need a digital mode set to send audio from the second RX out. Thetis I believe also can listen to two RX at the same time.
To utilize all 4 slices at the same time you may need to look at something like openwebRX. I'm not sure about SparkSDR.....