Ok so I may have found a possible solution. I originally didn't think this would work as I assumed this only applied to Source/Valve's games but it seems to be working so far; I will do more testing to confirm the solution but in the meantime, here's what I did to get it to work:
Quote:To configure multiple clients to play from behind the same router, you may need to change one of the clients to use a different clientport number.
The default clientport is 27005 and you can only have one game with that port active on the same lan.
- Each machine must have a unique Steam account with different game licenses registered to the account.
- Each machine must have a different clientport setting.
- Each machine must have a unique Mac address.
- The router must be up-to-date.
- Your Firewall and router must have the required ports for Steam open.
To specify a client port, please perform the following steps for each computer on the LAN
NOTE: This solution only works if you launch the game through the My games menu.
- Go to Steam's My games menu
- Right-click on the game you wish to play
- Go to Properties
- From the General tab, click on the Set launch options button
- Add a different client port number on each machine between 27005 and 27032 in the following format:
Note: Do not use 27020
- Click OK
If you have already set other launch options, leave a space between the last character of your previous launch option and the + of your clientport setting.
Link to the article is here https://support.steampowered.com/kb_article.php?ref=6370-QPHJ-3567