Say I have 2 fields in a table, f1, f2. Assume I want f2 to mirror the value of f1. How can I propogate changes from f1 to f2?

I've tried using record-level validation. I do something like:
IF OLDVAL(f1) != f1
REPLACE f2 WITH f1
ENDIF

This works, except the documentation seems to say I shouldn't do this.

Help.
Bob