So long story short, I'm DMing for a group of only two PCs. My idea was to have the players join a group of mercenaries to escort a dignitary into a dungeon. Each of the NPC mercenaries would basically be a DMPC, but I would have the real PCs choose which ones to interact with and become friends with. When they enter the dungeon, the mercenary band will split up, and the PCs could choose one or two DMPCs to join them. Later, everyone will meet up for the "final battle", where all but two of DMPCs will be killed off, but the PCs will be kept relatively intact. Then as they take the Macguffin, it'll set off a spell that traps all but one PC. Then a dilemma will occur with one DMPC demanding that the characters destroy it, and the other begging them to save it. Depending on which they choose, they'll have to fight one of the DMPCs as a sort of final boss battle.

Okay that rambled on a bit, sorry about that.

tl;dr: Need some tips on designing a short campaign (specifically encounters) for only 2 PCs?

Also, this is my first time DMing, so any other random advice would be cool too.