There are only answers not to use a VPN tunnel. In principle I agree with that because I think using a virtual private network on a simple local area network without public access for a simple secured point to point connection is overkill. But to answer your question for an OpenVPN you can look at Cannot connect to PiVPN Server from WAN but can connect through LAN. There you can find a simple setup mostly thought as entry point for setting up a VPN. The diagram shows an VPN tunnel through the internet but the first step of the setup is for local testing so you should be able to adopt it for your needs.

There are only answers not to use a VPN tunnel. In principle I agree with that because I think using a virtual private network on a simple local area network without public access for a simple secured point to point connection is overkill. But to answer your question for an OpenVPN you can look at Simple openVPN with static keys. There you can find a simple setup mostly thought as entry point for setting up a VPN. The diagram shows an VPN tunnel through the internet but the first step of the setup is for local testing so you should be able to adopt it for your needs.