Question / Help Dual PC Streaming with nginx-rtmp (no capture card) - Question

I just discovered this amazing method of dual PC streaming with no capture card, but I have a little question for those who are experienced on this.

Is it possible to have a stable connection between Gaming PC and Streaming PC through the router?
I mean, both my PCs have only 1 ethernet port, so it's not possible to connect them directly.

I did some tests and it worked well, but sometimes the image froze and got all blocky. Is this possibly because there's already internet traffic in these ports? Should I get a separate ethernet card for each one?
 
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