I am not sure how you were able to get an output from the second card, but when you have 2 cards in Crossfire mode, the outputs from the 2nd card are turned off and it is only used for rendering.
I'm not sure either, but it worked like that for 5 years until just the
other day. Eventually I got it working through trial and error, but
unfortunately I don't have anything to report back as to what it was that I
did that actually fixed the issue.