“Use two audio devices” can describe two different jobs. Recording separate inputs from multiple interfaces requires a different arrangement from playing the same sound through two outputs. Write down the intended channels before changing Audio MIDI Setup.

For example, list which microphone should appear on which input, or which two speaker destinations should reproduce a playback test. Start at a low listening level and save the current application audio settings so you can return to the known arrangement.

Choose the matching device type

Apple describes an aggregate device as combining several audio devices into one device with their available input and output channels. In Audio MIDI Setup, use the Add button, choose Create Aggregate Device and select the component devices with their Use checkboxes. Give the combination a descriptive name.

A multi-output device serves simultaneous playback. Choose Create Multi-Output Device, select the required outputs and choose its primary device. Apple explains that audio sent to the primary is also played through the other selected devices.

Clock controls vary by macOS version. In the macOS Tahoe 26 guide, aggregate devices require matching sample rates and appropriate Clock Source and Drift Correction choices. Follow that version’s guidance together with the interface manufacturer’s hardware synchronisation instructions. Do not copy newer Sync Source controls onto an installation that shows different options.

Test the route inside the application

Select the new device in the intended application where supported. For recording, make a short harmless test of each input separately and inspect which channel receives it. A combined device appearing in a menu does not prove the channels are mapped as expected.

For playback, confirm sound reaches each intended output at a comfortable level. Listen for timing differences and check whether the arrangement suits the actual task before using it in a presentation.

Keep the test small and reversible. If the application cannot use the required channel arrangement, return to the original device and record the limitation. Combining devices does not by itself add an audio-routing feature the application lacks.

References: Apple: aggregate devices, Apple: multiple outputs, Apple: Tahoe aggregate settings.