Which one should I buy?
Adventure pals or Them's Fighting Herds?
Of course, bought at humble store.

Adventure Pals seems like a more interesting game.

I don't know Them's Fighting Herds, but I do know Adventure Pals, so if you have any questions about it that could help you make up your mind, feel free to ask and I'll try to answer them.

You have gog's version, right? Does this have co-op achievement or in-game achievement? How hard is to 100% the game with two controllers, but only one real player?

Oh, these are not the kind of questions I expected ... I only have it on Steam and I haven't 100%ed it yet. As far as I can tell, there are no in-game achievements, just the ones from the client. There is one achievement about starting a co-op game, but apart from that it doesn't look like you actually need to play the game in co-op mode to get all achievements.

Thanks!