If my friend would share to my account and to HIS alt account at the same time and he plays from his alt where I play from my main account, can we play at the same time over a Steam account?
No. Only one person can use a given shared library at a time. If it's shared with multiple people, only one of them may use it at a time.
Can two users share a library and both play at the same time?
No, a shared library may only be played by one user at a time including the owner and even if they want to play different games.