Hi, I know this is an old post… I am sort of doing what you suggest… I’ve been using the UR824 for a while now (love it, rock solid) and wanted to add additional inputs. I considered the Behringer option or a Presonus digimax but decided to go for a UR816C, use it as my main interface with the UR824 connected via ADAT for an additional 8 inputs. As mentioned, the UR824 does not have MIDI while the 816 does.
Couple of things I’m not sure about… It was mentioned that Cubase only accepts one interface at a time yet as you said, Steinberg clearly states that two 816’s can be connected at a time, although I’m not sure in what capacity.
So would my UR824 would no longer be connected via USB? only to the ADAT connections on the 816?
Still trying to sort this out… stay tuned!