Well, you need appropriate speakers with two sets of terminals. There will be some sort of connection to be used when not bi-wiring or bi-amping. You don't want to connect two amp outputs together. Read the speaker manual.
You need a second set of cables. Read the cable manual (joke).
I don't know your Denon but I have the X3200W. On mine, the usual front outputs go to one set of terminals on the speakers. There is another set of outputs which have multiple uses: e.g. 7.1, Atmos, zone 2, or bi-amping. Finally, you tell the amp that you want this. Again, read the manual.
I tried it once just because I could. I had two channels doing nothing. Was it worth it? No, not for me anyway, I could not tell any difference. I was a bit disappointed that Audyssey did not calibrate the two channels independently. I was also surprised that it was not specified which outputs went to which speaker terminals.
Will i do it again? No, partly because I could not detect any benefit but also because I bought some Atmos speakers and I no longer have any spare outputs. This is a much better use of the extra outputs. 7.1 is also good but I prefer Atmos.
Not surprised. What you are really doing is passive bi-amping taking the power from the single PSU in the Denon. That power has to come from somewhere and it's is taken from the remaining connected speakers. You are robbing Peter to pay Paul.
I had hoped that Audyssey would calibrate the two channels independently. I expected that this would allow more accurate calibration. That it didn't, together with Gibbsy's point convinced me that it was pointless.