So: lets say, I (player A) and my friend (player B) both own skyrim on our steam account, but we decide to family share, will me and him be able to play skyrim at the same time? also (diferent question now) if i we family share and i want to play gmod and he wants to play some other game skyrim for example, would it be posible to do it at the same time since they are diferent games?
The family sharing page of Steam has a FAQ with useful information.
Can two users share a library and both play at the same time?
No, a shared library may only be accessed by one user at a time.
When I authorize a device to lend my library to others, do I limit my own ability to access and play my games?
As the account holder, you may always access and play your games at any time. If you decide to start playing when another user is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing.
That means that both of you will be able to play Skyrim, since you can access your own games at any time, but you can't play a shared game if the owner of the library is currently playing any game.