I don't have any experience with Focusrite or Presonus but I use the adat connections on my MOTU without any problems; generally speaking, the expansion unit doesn't connect to your computer, it only connects to the main unit by the adat cables. So then you have to set everything up using the control app for your main unit so that it receives the adat channels and sends them through to the computer. Then in your DAW they just look like additional channels from the main unit. So if you can't run the main app for your Presonus then it might not be possible to set up. Not sure what the issue would be with using two Focusrites though. I will say that the exception to not connecting the 2nd unit to the computer is sometimes you may have to do that to use it's control app to tell it to route out the adat ports.