Combine audio devices to use a single device in Audio Midi Setup on Mac
You can combine several audio devices to use as a single device, known as an aggregate device. By aggregating devices, you can increase the number of discrete audio inputs and outputs without purchasing more expensive multichannel audio equipment. You can use the aggregate device for sound input, sound output, or for alerts and sound effects.
For example, if you have an eight-channel audio device and a two-channel audio device, you can combine them to work as a single ten-channel audio device.
For the aggregate device to work correctly, the combined devices in the aggregate device must be set to the same sample rate.
In the Audio MIDI Setup app
on your Mac, click the Add button 
in the Audio Devices window, then choose Create Aggregate Device.To rename the aggregate device, double-click it.
In the right side of the window, select the Use checkbox for each device you want to include in the aggregate device.
Next to the audio device names are the number of input and output channels for each device.
If the Clock Source pop-up menu is available, you can use the clock of one device as the master clock for all the combined devices.
To remove an aggregate device, select it on the left side of the Audio Devices window, then click the Remove button 
.