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How can I use more than one audio interface on Windows?

Most professional Digital Audio applications on Windows use the ASIO (Audio Stream Input Output) system for low-latency multi-channel audio. This is a special audio driver originally created by Steinberg that is optimized for pro audio use. Unfortunately because of the way it is created on Windows, ASIO is only capable of supporting a maximum of one audio interface for input and output at any one time. This means that it is usually effectively impossible to combine or "aggregate" two or more professional audio interfaces when using Windows. 

Since many Digital Audio Workstation applications can only use the ASIO system for audio I/O, they cannot support any other audio driver either. For example, this is what Ableton have to say about using Live on Windows:

Aggregate devices on Windows

On Windows it's not possible to create aggregate devices at a system level. If using the MME/DirectX, you can select two different devices for input and output (like on Mac). If using the ASIO driver, it's only possible to select one device as both input and output in Live's preferences.

If your DAW can support Windows Driver Model (WDM) input and output, it may be possible to use this to connect to multiple interfaces - however because WDM has traditionally been found to be buggy when used for multi-channel audio this is not supported as a professional solution by either us or most other manufacturers.

Another workaround is to use the infamous ASIO4ALL application. This basically hacks the ASIO audio stream and forces it through the Windows WDM driver subsystem instead of sending it directly to the audio interface. By doing so, it can enable the use of multiple interfaces. However ASIO4ALL is not supported either by us or most DAW manufacturers, and has not been updated since 2017; so if you do try it, there is no guarantee that it will work properly on your system. It may give rise to additional latency, or unexpected artifacts in your audio during recording and playback. Despite this, many users have reported success with it.