I agree with Patrick's assessment of stacking doomed keywords (don't know that there's an official ruling, though).
However, surge IS additive. Here's what Nate French had to say on the matter (from http://www.cardgamedb.com/forums/index.php?/topic/1702-offic...):
Question Submitted by "guciomir @ FFG"
Q: - In Hills Of Emyn Muil quest, treacheries can gain extra surge effect if there are no locations in play. What if I draw a treachery like impassable chasm which has its own surge effect? Does surge stack so I have to resolve 2x surge and draw 2 more cards from encounter deck? I suspect that this may be a case because as per FAQ, some effects like Caught in the web are stackable (and this situation looks similar to me). Can you please confirm?
A: - Yes, If a card has multiple instances of the surge keyword, each instance should be resolved.
I would posit that guarded would follow the same pattern, but I don't know of anything concrete to back that up.