I had asked about this previously in this thread:
At the time I was using Digitakt OS 1.10, with an overbridge beta I got from Facebook. I got the audio aggregate device working with all my other sound cards that are not the digitakt. Every time I tried to add the digitakt to an aggregate device, it would slow my machine down like crazy - and selecting that aggregate device in Ableton would make Ableton slow down or crash.
The consensus on Facebook is that audio aggregates are not a completely trustworthy way to go.
I hope we see this need for aggregates evaded by overbridge 2.0!