Can I use two interfaces at the same time for more I/O? (aggregation)

Yes, macOS allows you to use multiple audio devices at once and use them as-if they were a single device. Through a process called Aggregation, inputs and outputs of the separate devices are combined and can be used simultaneously.  For example, you can aggregate 2 ONE's, effectively creating a device with 4 output channels.

Note this is different than the multi-unit mode used to combine two Apogee Element or Ensemble Thunderbolt units together, which uses Apogee's custom Thunderbolt Driver to enable more sophisticated features and capabilities than Apple's Aggregate Device feature.

Apogee generally discourages device aggregation for critical professional usage due to clocking issues and other audio artifacts that can occur.

For those that wish to try it anyway, we recommended limiting to a maximum of 2 devices only on macOS.

  To create an aggregate device and use multiple interfaces simultaneously:

• Open Audio Midi Setup (Located in Applications/Utilities/ on your Mac)
• Click the + from the button left of the window
• Select "Create Aggregate device"
• Double-click the newly created device's name "Aggregate Device" and rename it if desired
• In the right side of the window, select the devices you wish to aggregate. For example ONE, Built-in Input, Built-in Output.
• For best results, be sure Apogee device is selected as the Master Clock
• In your recording application, select the device name you've just created.
• To select this device to be your default for Mac sound, Right click (or control click) the device in Audio MIDI Setup and select "Use this device for sound input/output"
• Select the Output speaker channels by clicking "Configure Speakers", selecting the stereo tab and assigning destinations for left and right channels
• Select "Done" button and you're all set.

Here are the instructions Apple Support provides: https://support.apple.com/en-us/HT202000