frap_vienna.rb

class User < Vienna::Model
  attributes :first_name, :last_name
end

user = User.new
first = Frappuccino::Stream.new(user).capture(:changed_first_name)
last = Frappuccino::Stream.new(user).capture(:changed_last_name)

# last name is any change to first or last name
last_name = first.merge(last).map { "#{user.first_name} #{user.last_name}" }

user.first_name = "Ford"
last_name.now # => "Ford "

user.last_name = "Prefect"
last_name.now # => "Ford Prefect"