Play audio through multiple devices at once in Audio MIDI Set-Up on Mac
If you use more than one audio device to create a multi-output device, you can play audio through several devices at once. For example, when you add two devices to a multi-output device, audio sent to the primary device also plays through any other device in the stack.
In the Audio MIDI Set-Up app
on your Mac, click the Add button 
at the bottom of the sidebar in the Audio Devices window, then choose Create Multi-Output Device.A new multi-output device appears in the sidebar. To rename the device, click it, then enter a new name.
Select the new multi-output device, then on the right side of the Audio Devices window, select the Use tick box of each device you want to use in the multi-output device.
To set the primary device for the multi-output device, click the Primary Device pop-up menu, then choose an output device.
To remove a multi-output device, select it on the right side of the Audio Devices window, then click the Remove button 
.