Half life 2 syngergy mod lan

Hi I was wondering if I need two copies of Half Life 2 in order to play Lan with my gf. I really want to play with her but I don't want to waste money buying another copy. Thanks.

Well if she does't have it - yes you do. You can't both play from the same steam acc.

Is there no way to play offline. Like not connected to steam. You know how you can play CS without connecting to steam.

Run half-life 2 atleast once on both the computers, start steam in offline mode and profit. Anyways for Half-Life 2 co-op I'd recommend the Obsidian Conflict mod it's way better than synergy in my opinion.