Multiple FiveM instances open

I suppose my question/issue goes here. Haven’t posted here in a long time, so could be wrong.
So I’m developing a script for my FiveM server, but I need a second fivem client open at the same time to be able to test it. Is it even possible to do this?
What I tried:
Simply copy the FiveM folder.
Trying sandboxie because of this thread. I couldn’t get it to work, because it gave a privcache renaming error.

Info:

GTA V version? 1.0.1604.1
Up to date? Yes
Legit or Pirate copy? Legit
Steam/CD/Social Club? Steam
Windows version? 1903

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