I was wondering what are the best heroes for playing SoN? At this point we only have two heroes vs Overlord so which ones would be the best choice?

cheers

There are a variety of hero compositions that work well, but almost all have weaknesses that the overlord can exploit if he's playing well. If you assume that the overlord is competent and will punish weaknesses, then you want a group that's well-rounded or one that has the most irrelevant weaknesses.

What resources do you have to choose from? Do you know anything about the overlord's playstyle or what plot deck he's likely to choose?

You may have seen this elsewhere on these forums, but I would suggest that you try a 4 hero game, even if each hero player has to play two heroes each. It does wonders for the balance of thegame.

And I'll second what Whitewing said: There are a variety of composition of parties that work well. I've found that having one of each archetype is a good start. You want a tank, a healer, a really mobile character and someone with crowd control. Select your characters so their attributes complement each other. Be aware of which characters have attributes the OL can punish and be ready to support them (IE: almost every mage vs the poisoned condition)

Having recently OL'd a Nerekhall campaign, I really suggest not playing with 2 heroes, though you are welcome to try. The heroes had a slight edge on me for most of a 4 hero campaign, the finale came down to the wire, and I was facing a solid hero party:

Syndrael the Knight

Mok the Bard

Tinashi the Shadow Walker

Dezra the Necromancer

If you are going to play a 2-hero game, I would actually recommend Tinashi the Shadow Walker. There are a few quests where teleporting through a portcullis at a key moment is very important, but she's also rather tough hero as far as scouts go. (Shadow Walker is an effective scout class, but you may want to go Treasure Hunter or Wildlander instead.)

Since Tinashi has your scout/warrior angle sort of covered, you're going to want your other hero to be more mage-like, I would assume. It also certainly wouldn't hurt to have a healer, as I promise you'll be getting damaged a bunch.