If you are trying to find the rule about this using only the compendium, the closest thing you're going to get is the glossary entry for
Off-Hand - which contains this text:
"An adventurer can’t attack with both weapons in the same turn, unless he or she has a power that allows such an attack, but he or she can attack with either weapon."
But seriously, at least drop the $20 on the Rules Compendium so you have a real rules reference at the table. It is well laid out and actually has a fairly comprehensive index, unlike the PHB. Some would say there are errors in it but WoTC stands by its text as the definitive rules source for 4e.
