Dual PC System

Before Getting Started

This guide assumes you already have 2 PC's each with their own monitor, keyboard, mouse and usb headset with microphone.
You will still do everything you do now on your Game PC, music, chat, gaming, but your streaming will be on another computer.
You can use your Game PC for just gaming with music, chat and stream on your Stream PC (basically what I do), but requires more indepth configurations.
If using a USB headset, it is possible to route all audio from Game PC to the Stream PC via the HDMI / DP

Having 2 PC's, one monitor, one keyboard and one mouse can be worked with the use of a KVM switch, but there will be keyboard / mouse "lag" when switching PC's. (What I started on)
Digital mixers like Voicemeter, Bannana or Potato are not needed. Both OBS Studio and Streamlabs, when openend, will act as your mixer for a basic setup. More complex setups will require a mixer of some sort, digital, physical or both.

Gaming PC
Streaming PC
USB Capture device or PCI-e Capture device
HDMI Splitter - Optional (Allows you to still get video to monitor when Streaming PC is turned off)

Wire video (no splitter): Game PC > Capture Device > Stream PC
Wire video (with splitter): Game PC > Splitter out 1 > Capture Device > Stream PC & Game PC > Splitter out 2 > Game PC Monitor
Wire audio (with usb headset): Game PC line out > Buzz Box > Stream PC Line In
Wire audio (with 3.5mm headset): Game PC headphones out > Buzz Box > 3.5mm Splitter out 1 > headset & Game PC headphones out > Buzz Box > 3.5mm splitter out 2 > Stream PC Line In

This will be the most complicated part of the setup, go slow and be patient.
Game PC: Set everything to the Audio Out (line out, headphones out or hdmi out)
Stream PC: Set Windows Recording to line in and Playback to default. Set OBS or Streamlabs for input to be line in.
Remember games and software have audio settings too

If you want to have audio from your Stream PC back to your Game PC
Not in Schematic: Add additional 3.5mm male to male cable from Stream PC line out to Game PC line in
Game & Stream PC: This will start to be a headache and because all systems are different, all I can advise is getting a mixer. Choosing a digital mixer from VB-Audio is a popular choice, watch their tutorials. Getting a physical mixer like a Go-XLR is also a popular choice but requires an XLR microphone. (Recommend a Shure MV7+)

Virtualize!!! Upgrade Streaming PC to Windows Pro and setup Hyper-V (built into windows - Free!) and host games, websites, developer platforms and more!
If Streaming PC is using an NVidia GPU, Install NVidia Broadcaster for AI assisted filtering and effects for Audio and Camera

NAS (Network Attached Storage). Add more SSD's and/or HDD's and create a shared folder on the network to record streams to and/or make it easier to swap files back and foreth from Streaming PC to Gaming PC.

If you're taking old hardware to get something together then I would say a Dual PC setup is worth it. Benefits of putting the extra load and CPU cycles onto another system to get the max benefit out of your Game PC or Production PC is a huge advantage. Being able to add other systems like consoles becomes very easy with a Dual PC setup as well with some extra hardware additions. Having a second system also gives you additional USB controllers between two systems. A USB Controller is what has USB ports connected to it and why when you have to many things plugged into one system, the notorious Ding happens and you lose things like a headset, audio, microphone, webcams shutoff. Powered hubs only work so well as you are plugging a hub into a USB port that already has a controller that is sweating bullets with activity. For me, I use a Dual PC setup to give myself more USB Controllers, let me Game PC go full power when also doing AI processing for Production work while my Stream PC handles streaming, music, discord chat, it is a NAS and virtualizes VM's when I want it to for trying out different software packages or even to host a game locally. Also, a bit of a side note, there is a 4k webcam connected to my Stream PC and a 1080P webcam connected to my Game PC so the stream gets the 4k and conferencing happens on the 1080p webcam. It is very frustrating setting up initially, but in my opinion, well worth it in the end.

Having a beast of a system will likely run gaming, production and streaming just fine all at the same time. Some folks have even talked about an improvement of the system. By sticking with a Single PC system, you'll only have one source of traffic on the network, less power consumption and only one keyboard and mouse to deal with. Creating less heat is subjective to how the system is built, it will have more load which will create more heat, so it really just depends on how it's built. A Single PC is way less stressful for setting up Audio and will save you $$$ in aspirin. Streaming on a Single PC system is also fairly easy to setup, click and go, however, if you lose connection your audience will most likely dip out as well. You are kind of limited for USB controllers (not ports, ports are connected to controllers on the motherboard) and if something goes wrong, everything stops. But, its simple and you wont feel like you're buying two of everything.