Two servers in local network

I have two Bitcoin Core running on: 1. Intel NUC 2. desktop PC. Local network, router, connected to the Internet.

What and how should I configure to make desktop use always-up-to-date blockchain from Intel NUC?

What I have done already is:

1. configure then start both (Intel NUC running always)
2. set different ports for both and port redirection on router, also upnp=1, port=..., and server=1
3. set rpc variables (seems like not needed)
4. on desktop set addnode to external IP with port to NUC's Core; desktop running Win10

Is addnode on desktop enough? Seems like not - it does not connect or lose connection to NUC. Both internal IP and external IP I tried.

Should I use local IP address and port?

Can you advise in the topic?