2 instance of postgres service running against same db files?

  Can I have 2 instance of postgres service running against same db???. I
meant 2 machine or the same machine with a 2 running executing postgres
process (if it is in the same machine, then through different ports),
working with the same db files,  may be in a shared storage or any common
mounted file system. Any other variant? How?

 Im sorry because of my english. I speak spanish.

 thanks...