Yeah, with 2 heroes, you really have to take into account a lot of different factors and choose the pair of heroes and their classes a bit more carefully.
Tips for playing with 2 heroes:
Classes with familiars treated as figures become more powerful (relatively speaking) because they increase the figure count by a higher percentage than when you have more heroes.
Pay VERY close attention to the attribute spread. With any number of players, you should try and make sure you have at least a 3 or 4 in every attribute so that at least SOMEONE can deal with any given situation. (This usually happens naturally in games with more players, but has to be very intentional in a 2-player game.) A key mistake is for players to not pay attention to these numbers when they choose their characters and suddenly realize that the entire party is weak in one attribute. As soon as the OL notices this, he can change tactics to use creatures and quests that exploit that weakness, while the heroes are stuck with them for the rest of that campaign.
Try and have one character with strong utility or healing and one with strong damage potential. With only 2 heroes, focusing on just one isn't going to give you quite the overwhelming tipping effect that can happen with a party of 4 heavy damage dealers or 4 heavy utility characters. Such gimmick parties can work well with larger groups, but can be really tough with smaller numbers. (As a side note, now that there are enough expansions out, I'm really looking forward to hearing tales of the single archetype groups: 4 warriors, 4 healers, etc.)
While a bit debatable, I feel that having a healer in the group is more important in a 2-hero game than otherwise because if the OL can take down one hero, you've suddenly lost half your party, and preventing that from happening should be a big focus. Along with that, as a healer in a 2-hero campaign, I'd focus on single-target healing instead of area of effect spells (and opposite for a 4-hero campaign).