I have one application says "app1"(main application support login) which is deploy some different machine tomcat server and another application says "app2" which also deployed in another machine tomcat server. So, should i need to install siteminder web agent to both the machine or it is on only in "appl2"?
You can use a traditional reverse proxy (apache with mod_proxy) or SiteMinder Secure Proxy Server to handle the Web Agent work and forward traffic to the destination web/app servers.
Secure Proxy Server enables your Single Sign-On environment to have "agentless" capabilities. You will still have 1 or more SiteMinder Web Agents (depending on the number of proxies that are deployed), but the web and application servers will not need to have any agents installed. The web/app servers only need to be able to consume the HTTP Headers provided by SiteMinder.