Duet does not support aggregate device setups. In most cases it will allow you to set it up and it will work normally for up to 20 minutes and then will start to have glitches until it crashes.
It depends on the other soundcard. The fireface does not support, mackie does, tomorrow I'm receiving my new metric halo uln2 and will let you know!
On the uln2 you can go wrong. The main reason is that it is a very very high quality A/D converter and
It has two professional mic pres, perfect for me (singing is my main role) and to record acuistic ( second main thing)!
But mainly it is very simple to use and has a powerful software MIo console.
If it is used in studio there must be a reason