Is Road to Legend Scaled so that even a small party of 2 Heroes can survive (2-player game) and have a fighting chance during the entire campaign?

No, everything expects 4 heroes. It would be far less work to have one player running 4 heroes than it would be to re-balance everything for 2 heroes, in my opinion.

This isn't too big of a deal since the base game is best with 4 heroes anyway. Anything less and the heroes have a pretty rough time.

You really need 4 heroes, especially for RtL (I hear you can get by with 3 in vanilla, I've only done RtL so far myself).

If the person playing the heroes is up for it, I'd recommend they just play all 4 heroes, that's what I'm doing right now with my brother over video chat. Works better than one would expect. Works out a little different than it would with more people, as it's just one mind coordinating how best to use the 4 heroes on each turn. Has its ups and downs to it, as only 1 person won't see as many angles on how to approach the turn, although he doesn't have to work with negotiating with the others on what order to go in and who should kill what.