It will be a very short but helpful guide.
To achieve the hardest achievement in 2025+ you need to:

1. Find another one player that have at least 2 characters
   
2. Open steamapps\common\FightTheDragon and start two times FightTheDragon.exe. In both games select different characters.
   
3. One of you should make host and connect his second character. Other one should join with both characters.
   
4. Play one easy map. Leave mules at the start position.
   
5. Clear the level and then return to the home.
   
6. Done.

If the achievement did not pop then try to disconnect from the party and check if it did appear in the steam's profile.

If someone wants to finish this with my help, just write me a message. The game is awesome, but sadly was abandoned.