I recall that it's added damage of elemental type, which is supported [here][1]

The [Dragon Age Wiki][2] states that multiple of the same type also stack

According to this [thread][3], even same typed stack such as Frost Weapons spell and frost runes

So you could do two +2 frost, and frost weapon spell, and they would all add up on top of your normal weapon damage.


  [1]: http://dragon-age.alteredgamer.com/dragon-age-origins/58594-doa-tactics-runes-and-enchantment-guide/
  [2]: http://dragonage.wikia.com/wiki/Weapon_runes_%28Origins%29
  [3]: http://www.gamespot.com/dragon-age-origins/forum/morrigans-frost-weapon-buff-stackable-with-runes-52156763/