I have only ever played this game with my friends.

And I have always only use 1 damage deck. However for some reason now I am thinking I read some where that you are supposed to use one per side. Can anyone clarify this for me please?

It is part of the Tournament Rules.

Each player is required to bring their own components, including a Damage Deck.

HOWEVER

For small games, or just amongst friends... Its not really a big deal. It has the potential to skew some things, but it at least skews it equally somewhat...

Because for the most part, "Tournament Rules" is what applies to anything that's outside of just the Learn to Play and Basic rulebook... I mean, as it is, if you're playing Rulebook games, then you're probably playing 300pt games, because that's what is says...

I started with two core sets so I've always had enough cards for each to use their own. I don't see much wrong with only using one deck though

Each player needs his own damage deck.