Run two fivems?

Is it possible to run 2 fivem clients connected to like a localhost for testing or is that not possible?

Yes, that’s possible but you should use Running FiveM in a Hyper-V VM with full GPU performance for testing ("GPU Partitioning") instead. If you’re going to go with just running two clients without hyper v then make sure you remove svadhesive and adhesive from your game and server directory.