Same user need to complete two user input tasks or same user unable to act onthis

in a process i have two  user input tasks let first is x and second is y

now i have scenario

1) where the user 1 completed X then he only need to complete Y

or where the user 2 completed X then he only need to complete Y

2)when the user 1 complete X then we have to restrict to complete Y by  the user 1

or 

when the user 2 complete X then we have to restrict to complete Y by  the user 2