I haven't played Divine Divinity more than some two hours, and I intend to skip Beyond Divinity due to its poor reception. Can I still start playing Divinity 2 and expect myself to be brought up to speed in the plot and setting, or are the first two so important to the plot? (Imagine playing Mass Effect 3 without having played either 1 or 2).

You will find some in-game references to the first two games, but Divinity 2 is a very different game to Divine Divinity and Beyond Divinity. For instance you play from an over the shoulder perspective rather than an isometric one and the graphics are poles apart. Fights can be hard and winning some of them does require thought. Also, this is a much more open world than the previous two. I found these elements made Divinity 2 a much more involving and exciting game. Plus, once you reach a certain part of the game you can fly and fight as a dragon.

The introduction does give you some insight, the in game references are not that frequent and you are playing the game from a different perspective to what is offered in Divine Divinity or Beyond Divinity. So you should be fine playing Divinity 2 in isolation and I hope you enjoy the game - it is my favourite of the 3.

the game is filled with books that tell you the plot anyway, although obviously they retconned some of it.

I played it start to finish without playing the first two. Obviously, I may have missed references to the other games, but really I did not feel lost or confused at all. The game appears to stand completely on its own. Any references to the other games are the type noticed by players of the other games and not noticed by those who jumped into Divinity 2.

You can read the plot in a walkthrough/faq or wikipedia, learn about the world and the characters, read about the ending etc. Of course it is not the same like actually playing the games but I think it'll work for you.

But out of curiosity, why did you played the first one, only for two hours?