You may have tried to set up an aggregate device on your Mac using our video here but still have no joy in getting it to work.
One thing that some people miss is the fact that when you set up aggregate devices that a core sample rate is set to ensure that all combined devices have matching sample rates.
If you then try and use the aggregate device in a DAW and set the session sample rate at, for example, 44.1k and your aggregate device has a sample rate of 48k then it will not work. You session rate must match the sample rate of your aggregate device. For many it’s a simple as that!