You can combine multiple ml:1s using the Aggregate Device function of OS X. Please follow these steps:
- In your Utilities folder on your Mac you find the AudioMIDI Setup application.
- Open it and click on the + sign in the bottom-left corner of the app.
- Click on „Create Aggregate Device“, name it and select it.
- Enable the checkbox „Use“ for each ml:1 you want to add.
For further details, follow the link to the Apple Support Page: https://support.apple.com/en-us/HT202000