If you use boolean you have to include IF action to your project.
You have two options: you either click "use random login' or not.
So you check in IF action if variable "{-Variable.nick2-}"== "false"
And if it's true you fill textfield only with {-Variable.nick1-}
If result of IF action is true you put {-Profile.NickName-} to viriable1 and fill text filed with {-Variable.nick1-}
You dont use both at the same time.