Aggregate device
A virtual audio device in macOS that merges several physical interfaces so an application sees them as one.
Most DAWs can only open one audio device at a time. To record from two audio interfaces at once, you create an aggregate device in macOS's Audio MIDI Setup, and Core Audio presents the channels of every member device as a single long list.
The hard part is clocks. Two interfaces that both believe they run at 48 kHz will still drift apart by a few ppm. One device is therefore nominated clock source, and the others can have drift correction enabled, which resamples them on the fly. Locking them together with word clock is the better fix. Windows has no equivalent in the ASIO world, where one driver at a time is the rule; on Linux, PipeWire and JACK do the same job.