Offtopic: I love Audio MIDI Setup for the Multi-Output Device and the possibility of connecting two Bluetooth headphones to one laptop. Is there a similar thing for Windows or Linux?
Many Linux distributions have moved to Pipewire as the default audio daemon in recent years. With Pipewire, you can use Qjackctl or Qpwgraph to edit how devices route to eachother using a visual graph. Pipewire has the distinct advantage that it works with tools for both the PulseAudio daemon and the JACK daemon simultaneously, so you can typically combine things you might do with either.
Unfortunately, I don't think there's a good UI that exposes both a graph view and a way to create virtual devices for more advanced routing, however there is very rich support for advanced routing:
So there's probably no direct analogue, as at best you'll probably need to run some commands to get a device to combine streams for you, and then you can hook it in using Qpwgraph.
Pipewire also handles video devices and graphs too.
If you ever want to try this with a proper audio interface, you'll also want to switch the audio interface into Pro Audio mode most likely. It's a profile that can be selected (typically in whatever handles your volume settings, e.g. pavucontrol)
