You could sort the elements before sending it to Kaminary, like this:
@things = @friend_things + @user_things
@things.sort! { |a,b| a.created_at <=> b.created_at }
@results = Kaminari.paginate_array(@things).page(params[:page]).per(20)
or
@things = @friend_things + @user_things
@things.sort_by! { |thing| thing.created_at }
@results = Kaminari.paginate_array(@things).page(params[:page]).per(20)