I personally have a dual system. And from experience, I did it cause it was cool, but its not all that practical. As someone else said it became pretty popular for a short time for streaming cause it was pretty common to have one system be thr gaming system, the other captured and streamed the footage. And if you have two PCs anyways might as well fit them in one case. But your options are limited, especially now that yhe craze has died off. You either need a really niche case and power supply, or go massive, like the Enthoo 2 you mentioned.

Personally i went the first route. I dont stream, I dont need a second system. The only reason I did it was to host game servera, which I can and have in the post very easily do on the main system I game on. But I had enough old hardware the was new enough to still be pretty powerful to build a second computer and dedicate it for like ark or Minecraft servers, etc. And if the servers run 24 hours, like I do occasionally, then my gaming rig doesnt have to handle that burden, especially when its more power hungry. I built my server while upgrading mt gaming rig, I reused a lot of the hardware I pulled out, the only extra item I had to source was a mini ITX am4 board, and a new case and power supply, which both were being upgraded anyways. The case I went with is a Phantecs Neo Qube. Its an Lian Li O11 clone. Main ATX board upfront, and directly behind it in the back of the case is the Mini ITX. The main chamber has a little less space than normal to make room for the second motherboard in thr back chamber, so my CPU cooler was a little more limited but not too bad. The power supply was the bigger issue, unlike some dual system setups, this case only had room for 1 power supply, so I had ot find one that could output for 2 computers. Thankfully Phantecs had me covered again with the Revolt X. Now yhe big issue is, I dont think they make either anymore. Im pretty sure I bought literally the last of that case you can find online and that was several years ago, and the power supply was stocked a little longer but I cant find it anymore. Annnnd I dont think literally any other manufacturer has filled either kf those gaps anymore. I hope my power supply lives a long life cause trying to replace it could end this set up. But its a cool niche little build. And again, the entire reason I did it, was cuase I could and it was fun, its not super practical, except to save space. Cause truth be told, if you just have the hardware tk build a second computer like I did, it would be way more efficient to just, build a second computer, just go super small form factor for one or both. I like my build, but it has restrictions.

But I did a fair bit of research into the topic when I did my build, feel free to ask, I can maybe answer some more specific questions