You can create Aggregate Device to increase the number audio inputs and outputs available at the same time. You can also use an Aggregate Device if your application doesn't support separate input and output devices.

With Logic Pro, GarageBand, and MainStage, you can use a separate device for input and output. For example, you can use a USB microphone for input and the headphone jack of your Mac for output. Some applications not manufactured by Apple don't support separate input and output devices.