How I can configure 2 network connection in 1 desktop computer?

Please anybody know how to configured two network in one Desktop computer like 1. Intranet for LAN and 2. for Internet connection? We have two network connection in our organization 1. for Intranet only for inside organization and 2. for Internet for sending email for outside the world. How I can use or configure the two network in one computer in one time? Like when I log I can choice internet or Intranet?

Thank you very much.