This is a guide explaining real quick how you can get the multiplayer achievements completely alone in 2019, seeing as the multiplayer is dead.

1) Find your .exe for Dragon commander. This should be in: Programfiles (x86) > Steam > steamapps > common > Divinity Dragon Commander > Shipping. Here you need to open "DCApp.exe" which will work as the other players for multiplayer.

2) Launch the game through your Steam. Click on multiplayer and then LAN. Create a lobby and join it with the non Steam version of the game. Remember to open the game 2 more times if you want the 2 v 2 achievement.

That's all! Warning: This makes the game really really laggy.