You can have as many messages as you want in the one script. What you CAN'T do is have IDENTICAL events in the same script.

Since you already have an ON JOIN event, you can add to it, but you can't put in another ON JOIN event.

If you're going to add to it, I suggest you line the coding and use braces, rather than having everything on one line

Eg: Current
Code:
 on me:*:join:#pokenightmare: .timerautomsg 0 20 .notice Hello and welcome to #pokenightmare.

New
Code:
on me:*:join:#pokenightmare:{
 .timerautomsg1 0 20 .notice $nick Hello and welcome to #pokenightmare.
.timerautomsg2 0 20 .notice $nick Chat rooms work better when people talk, so don't forget to talk
}