Actually, the longest campaign I ever played in was with 2 players and the DM--4 years. He wove NPCs in and out of the campaign, including a woman who fell in love wihi the other PC--it was really fun!!
I think a campaign with 2 players can be fully viable, and neither one of us wanted to play 2 characters. To this day, I won't as a player, and I don't allow it as a DM. As a DM, I adjust the story and encounters, provide items here and there that may serve some purpose (a barbarian shaman providing healing herbs that must be boiled in water, and taste awful, in gratitude for being rescued to help make up for the lack of a cleric, for example).
Be creative and imaginative in a way that helps bring out the flavor of your world also, and have fun with 2 characters, I say.