Then I tried the same thing with my newer laptop that is connected to my home network via etherner cable. Everything was working, my laptop could see my main PC as NDI source and add it.

First I tried it with my older laptop that is connected to my home network over the WIFI. Hello, I just tried NDI for the first time yesterday. Of course I could be wrong, hopefully one of the devs will get back to you on this soon. So I'm thinking this is by design if not prohibited in theory due to buffers etc.

Both Output NDI at once, and Both trying to receive, you'd be pushing 1000Mbps, PLUS Overhead of about 25-40Mbps at least. The NDI output from my gaming PC to the Transcode PC is 280-335Mbps Average (Using 1080P 60FPS), if you are trying to do this over Wi-Fi, with those kind of numbers you'd saturate almost any good-great Wi-Fi router, especially if both were trying to TX/RX at the same time, because remember your 250Mbps output from one, is 250 Mbps input on another PC, for 500Mbps total throughput against the router, over Wi-Fi that is really pushing the limits, if you were to try to RX/TX both at the same time, i.e. I mean for me, I'm using OBS to stream from two PC's, basically using one to Transcode to twitch (5Mbps) and the other to send to Restream(12Mbps) which forwards on to 4 other platforms. If so, then I'd say everything is working as it should. Test to see if you enable it only on one at a time that you can see it in both places, again with only ONE PC set as output enabled at a time. Of course I could be wrong, this isn't my software and I just started using it myself, but it works wonderfully and is simple. I can't say for sure as I just started using this myself, but I would think by design that this would not be allowed as you could cause a circular loop and either which way it would gobble up your NIC/CPU in an instant and make everything choppy if even legible.

Obs virtual cable