I couldn't find anywhere that explained how to do this, and it is actually straight forward and really cool, so I thought I'd share a step by step method that works for me, in case it might help someone out there:

1. Find the local IP address of the computer that will host. I do this by opening the command prompt (search for "cmd" in the windows search bar), typing "ipconfig" and then it is listed as the first IPv4 Address

2. On the game page on GOG galaxy 2.0, click the little options button just right of the "Play" button

3. Scroll down to "Additional executables" then click on "Run Multiplayer Game" from the drop down menu

4. On the host computer, select "Start Network Host Game" then Start game. Make sure this computer loads the game first.

5. On the other computer, select "Start Network Client Game" and type in the hosts IP address you found in step 1 then click "OK"

6. On each computer, in the game menu, go to "Flight", then "Two Player". Select "Network" as the connection option. Make sure both computers select the same channel

7. The host computer should then choose the game options and start the game!

8. Have fun! While you can't play the campaigns, you get a decent selection of single missions that you can play as pilot/gunner in the same helicopter, or you can each fly a separate chopper together.

I haven't played over the internet so I'm not sure how that would work (if possible at all)