user.rb

class User < ActiveRecord::Base
  attr_accessor :current_phase
  
  def self.thereafter(phase)
    -> do
      current_phase.nil? or current_phase >= phase
    end
  end
  
  validates :email, presence: true, if: thereafter(1)
  validates :name, presence: true, if: thereafter(2)
end

users_controller.rb

class UsersController < ApplicationController
  def first
    user = User.new(params[:user].merge(current_phase: 1))
    
    if user.valid?
      #
    else
      #
    end
  end
  
  def second
    user = User.new(params[:user].merge(current_phase: 2))
    
    #
  end
end