If I want to play d2 over the net lan to lan and one lan has 2 pc's with diablo 2 on them both how should I setup the router so I can play with both pc's on the lan behind a router?

I dont see any reason youd have prob.
Who ever is gonna host the server, forward that port to it.
The 2nd pc shouldnt have a problem connecting to the LAN ip because its already behind the router. The outside guy conneccts to your WAN ip and is forwarded to your host. Congratzi. Should be fine.
Only time youd hit trouble is when you want 2 hosting behind the router, then youd have to find if the game supports you to manually change which port it listens on.
Or have the client change which port to connect to and have that forwarded to blahblah. you get the idea