Official comment

@Katey > No, you can use decks 1&2 when connected to USB B, just rotate the channel input selector to the appropriate deck based on what's currently in use and what's available:
So if the previous DJ is currently playing a track from the left deck on the mixer's channel 2, your first track would be loaded on your deck 2, which is the channel 3 input on USB B as circled. After the other DJ is done, you switch the channel 2 input selector to B as well.