So, I've just learned a great new trick for downloading images from multiple cards simultaneously. This is great when you come back after a day of shooting and have a bunch of cards to dump. In the past, I would do them one by one, but now that I know this trick, I can import as many cards as I have card reader slots.
The crux of the trick is that if you have multiple cards plugged into multiple card readers (or slots in a single reader) you need to choose each card from the Files pane and not the Devices pane. This way, you can select more than one folder source. (You can't select more than one device, that's why we have to do this via the Files area.)
Once you've Cmd-Clicked or Ctrl-Clicked to select folders from each card, you can hit the import button and go have a glass of wine while all of your pictures download from each of the cards.
You can see the entire procedure in the video below.