[man]include/man the first script into the second.
That's the easiest, and sometimes best way to do it. But there are others like:
[man]session[/man]s, [man]cookies[/man], hidden forms, URL GET variables.
If your first script is semi-static (i.e. relies upon no user-input) then you can just include the first script into the second; however, if the first script does require user-input, then it's a bit tougher. You'd need to utilize either sessions, or hidden forms or URL GET variables.
Hope that's a clear, not-so-technical answer 😉