Hey there & Happy new year !!!,
I think the best solution for your problem is to share the same library means having the same Lutris account for both users.
Then you not gonna need to install the game twice but just adding games manually in Lutris to get things work.
How did you do that ???
- You just need to redirect manually every game to there respective directory to get things work correctly.
I advise you to check them first in the user session where u did the original install to get the correct path & settings for the others users then your games should work perfectly.
Let me know if it helps.