how to use two submit_tag in a form ?

Not exactly what you asked for, but you could check params[:commit] in
the controller to see which submit was clicked.

<% form_for(:my_model, :url => my_models_path) do |f| %>
  <%= submit_tag "submit1" %><br />
  <%= submit_tag "submit2" %>
<% end %>

class MyModelsController < ApplicationController
  def create
    if params[:commit] == "submit1"
       # do something
    else
       # do something else
    end
  end
end