Connect 2 iMacs wirelessly
I have 2 2019 27-inch iMacs, I wonder if I can connect the 2nd wirelessly and use it as an extended monitor, or do I have to use an HDMI cable to connect it?
No at least not from Apple. You may want to use Luna and see if that is what you are after. https://astropad.com/product/lunadisplay/
Also, as a disclaimer I am not a Luna customer so I cannot endorse how well their products work.