It looks like the Focusrite has ADAT "out", and the Steinberg has ADAT "in". Should be easy to get connected between them.
Are you using Cubase with the Steinberg?
I'm wondering if the 8 additional mic inputs will then "show right up" with Cubase?
Sorry, I know nothing about Cubase. I'm using Reaper. A version of Cubase came with the Steinberg, but I guess I felt no particular need to download it and go through the trouble of learning another DAW. I'm kind of lazy that way, Cheers and Happy New Year
Reaper/Cubase/bla, bla, bla, anyDAW… It doesn't matter.
The real question is this: When 8 channels of ADAT are added, what happens? Do they show up as 8 additional channels over USB? That answer should be found the UR816C owner's manual. If it isn't found there, ask Steinberg/Yamaha Support.
I'm guessing Yes but do not go on my armchair guess. Find out for certain.
The next question: will the drivers you need to install (Windows or Mac) or Core Audio (Mac) support the resulting 12 or 16 channels? Again, the answer… BTW, if on a Mac, Core Audio does not support "Loopback function for streaming" but Steinberg drivers for the Mac should add that — as they do with other interfaces.
I recently bought an M-Audio Air 192/14 and had a similar question. The literature never addressed this question.. Tech Support told me 4 channels only over USB — enough, so I bought it. When I plugged it into my iMac Pro, I was pleasantly surprised to find out that each of the 8 input channels had a corresponding USB out in pairs 1–2, 3–4, 5–6 and 7–8. Wow — now I could plug everything in. When I later told this to M-Audio, the reply was, "That's great. I bet we could sell more if we told people." Ya think?
16x16 is a marketing term and means nothing—until you figure it out how that applies to you. Apparently, the 14 in my interface was 8 analog Ins, 4 analog outs + 2 headphone outs… In other words, that number was meaningless.
Another thing that's meaningless is USB3 over "Fast USB2" with these latest interfaces introduced since October 2019. The latency round trip difference between the two is around 1ms. Put your ears 18" closer to or further from from your monitor speakers. That's the real world difference.
I'm not knocking that Steinberg. It lacked two features I needed (MIDI and an In/Out Monitor Mix knob) which is why I didn't consider it. Yamaha designs and builds great gear. But, if deciding among all the available choices, look at the other features, not USB3 (or Thunderbolt 3) over Fast USB2.
Conversely, I wouldn't buy a new interface released before Oct. 2019. The newer stuff is better.