Hi Graham,
The requirement is that you use the latest version of Media Express which has an option to run in the background.
The cards in your system will enumerate in a fixed order but you can’t tell Media Express to address a specific card. It will pick the card which shows up as the first input/output device in your system. The next instance of Media Express you open will then pick up the second card.
Of course, the limitation here is that there’s no elegant way of capturing 2 streams simultaneously. You’ll have to start capture in one instance of Media Express, then go into the next instance and start another capture.
regards
Kristian Lam
Blackmagic Design