Starting processes as another user

There must be a simple answer to this one, but I can't think of it... When the machine boots up I want to start a program a twice - two instances of the same program, each running as a different user. These processes need to be running before anyone logs in, so I can just start them in the background from boot.local. The thing is, anything started from boot.local is obviously run as root. How do I set those processes running as owned by different users?