My general suggestion to people is: don't bother implementing netplay in emulators. You're entering a completely different world of completely different pain.
Without netplay, how should a homebrew NES game developer find testers for a 2-player game? Not everybody has other retro gamers in the family, especially those who are willing to put up with the repetitive nature of game testing.
Just post a download for the rom file, and I guess you can take someone's word for if they played it in 2 player with someone locally.
In the professional world, you can find a play tester in the same way you find other employees, usually. Job listings or relevant hiring agencies, combined with an offer of money for their services.
The other alternative is that a lot of people will test something for free in exchange for having some kind of early access to it (e.g. see Windows 10 right now). In this case the problem for you is generating sufficient interest in your project to make that exchange seem valuable to your potential testers.
The other problem with beta testers is they play how they want, you have a lot less leverage when asking them to do anything specific or in-depth. Paying people simplifies a lot of things.
Finding two-player testers is the same problem as finding one-player testers, just on a different scale. If you can afford to pay people, you can probably afford to pay them to use the same room.
I have no idea why you started this thread with an argument for netplay. That doesn't need a discussion. Of course it's useful for 2-player testing. There's nothing to argue in that respect.
With netplay, I could find random users in an IRC channel and test with them. Discouraging the existence of netplay, as koitsu appeared to recommend, shuts off that route.