models.rb

class User < ActiveRecord::Base
  [...]
  has_many :msgtos, :class_name => "Message", :foreign_key => :recipient_id #, :inverse_of => :from
  has_many :msgfroms, :class_name => "Message",:foreign_key => :sender_id #, :inverse_of => :to
end

class Message < ActiveRecord::Base
    belongs_to :to, class_name: "User", :foreign_key => :recipient_id, :inverse_of => :msgfroms
    belongs_to :from, class_name: "User", :foreign_key => :sender_id, :inverse_of => :msgtos
end