The problem with aggregate devices in macOS is, I think, that the different input microphones will be assigned to different channels. Zoom might pick up only the first channel of the aggregate device rather than mixing all the channels together.
Using a virtual audio device and a mixer software, one could mix the different microphones together into one channel of the virtual device and then select the virtual device as microphone in Zoom. Originally, I was thinking of soundflower (
brew install soundflower) or vb-cable (
brew install vb-cable), maybe in addition to obs (
brew install obs) for mixing. But as BlackHole has been mentioned in the comments, let's try this one:
BlackHole is a modern MacOS virtual audio driver that allows applications to pass audio to other applications with zero additional latency.
Install the homebrew package manager
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
Install the BlackHole virtual audio device
brew install blackhole-2ch
Install a mixer software that will be used to combine the microphone sources. You can use Apple Logic Pro if you have it, or you can use obs (free). Unfortunately, I didn't manage to get it working with Apple GarageBand, because this couldn't handle both microphones simultaneously. Here, I'm using obs.
brew install obs
In obs, in the "sources" section, add the first microphone as "Audio Input Capture".
Then add the second microphone the same way in obs.
In the obs settings (Cmd+,), under "Audio", under "Advanced", select the BlackHole audio device as "Monitoring Device".
In obs, for both microphones, you need to activate "Audio Monitoring". This will route the signal to the BlackHole device.
You find it in "Advanced Audio Properties" for the microphones.
Finally, in the Zoom settings, select the BlackHole audio device as microphone.
The volume levels of the two microphones can be mixed in obs. Zoom used this mix as input.