Hello, I've been using GIMX for the exact reason you've stated; playing local split screen games with my mates over Parsec.
The short answer is yes, you can plug in multiple GIMX adapters on a single PC. It becomes a bit of a mess of cables, wires, and managing program windows but it's totally do-able.
As it turns out, you don't need a custom firmware from that other thread (although that does keep things tidy), but what you do need to do is create and assign a new controller config per extra adapter plugged in and rebind all the buttons for that device. The default configs will be setup for Gamepad Xinput 0, make sure the new configs are looking for inputs from Gamepad Xinput 1/2/3...etc otherwise their controllers won't work or have input overlaps.
Now for Nintendo Switch specifically, I haven't tested the EMUSWITCH firmware myself; I believed it abandoned and no longer working. If it works for you, then cool but my method is using a Mayflash Magic NS adapter with its USB input. Feed it PS3/PS4 inputs from GIMX and it will work.
Alternatively you do not need GIMX or ANY adapters to achieve this on Switch, it has homebrew that can do it by sending inputs over your network. You will need a console with custom firmware.
Here is the guide for that:
https://gbatemp.net/threads/play-online ... ec.576705/
and here is useful homebrew for Switch that lets you use any bluetooth controllers:
https://github.com/ndeadly/MissionControl
And lastly, you will need a capture card with very little latency otherwise games will be unplayable. There's only two models that I would even consider, Elgato HD60 X or Avermedia Livegamer 4K. I use the Livegamer 4K myself and it's excellent. You will also need around 25mbps internet upload minimum to support 3 extra players.