Comentário oficial
Yes! I missed the post back in April, and then I had to get out my stuff in order to test after your post Friday.
No, you can't just show 2&3 in performance mode. I wish it were possible to re-number / re-order the decks, but unfortunately the GUI isn't designed to switch up. The best you can do is select the deck regardless of the player number:

And then show 4 decks in rekordbox:

But now your right CDJ (Player 2) is the lower left deck 3, and your left CDJ (Player 1), is the upper right deck 2.
On-air doesn't work with CDJs in Performance mode, regardless of the mixer used.