combine two audio inputs (microphones) for Zoom

I am trying to combine two microphones (usb & bluetooth, so no hardware option exists probably) to feed the combined audio to Zoom. I tried this with the Audio MIDI app (creating 1 aggregate device with the two inputs) of macOS and judging from the input level in system preferences it seems to work. Now I wanted to use this as the audio input in Zoom, however this did not work. It seems that Zoom somehow only took some of those channels effectively discarding one of the two microphones. Hence my question: is there some way to combine two inputs (ideally adjust their levels) and feed that to zoom? I have read a couple of similar questions (just without zoom reference) such as here , but they mostly to point to Audio Hijack which looks great, but I find it ridiculously expensive for such a little task.