This is how I've set up an aggregate device. I first selected my Focusrite (so it's ordered first) then the Blackhole virtual audio ports.
You can see in this screenshot that the first 2 output channels of the Blackhole will be the aggregate device's output channels 5 and 6 (the first 2 yellow ports listed under 'Output Channels'.
Then in S-Gear, you choose the Aggregate Device and then set routing like this:
- Input 1 is the first input channel on my Focusrite
- Outputs 5 and 6 are the first 2 output channels on the Blackhole virtual audio device.