I hate to chime in guys, and disagree with some of this, but i have tested 2 on 2 netplay with 2 LOCAL players, on one pc. It does not work using Retroarch. You cannot have 2 players on 1 pc , then link to 2 others on another netplay pc. REtroarch does not understand the 2 local players on netplay. I have tried and asked on certain forums. The only way you could do this is with 1 pc per player. Now with that said, i was able to play 2 local players on ZSNES vs one player on netplay. But in his situation, ZSNES isn’t an option. If all the people are in 1 room, your best bet is to mirror the monitor with a pi4 using two displays because it has 2 Micro hdmi display ports, and everyone BT their controllers inot the one pi 4.
So again, in short, you cannot start netplay with 2 local players in Retroarch, it kicks player 2 out. If anyone has evidence otherwise, I’d like to know.