The Time of Fire
About Companion NPCs
With only two players, the game could easily spin out of balance. So, your characters will have some backup during combat from your companions. The PCs will call all the shots, and won’t get a lot of help outside of combat. These NPCs will be mostly under your control, but they aren’t “you” in the game. These characters will join you when you need them, but you can pick up new ones as you progress through the game. For example, if you make peace with a tribe of trolls, you might get access to a new NPC that reflects this accomplishment: Grokku the Troll!
I’m looking at this article for guidance. I think this works well for now, but it’s still pretty new and untested. There are some issues: some of the NPCs have access to flight or invisibility, and monsters tend to be balanced differently from PCs (it’s a shame if the companion characters consistently deal more damage than the PCs, or if they end up having 2-3 times more hp). This needs to be tested; Can 12th level PCs match the numbers of 12th level monsters? Keep in mind that a 12th level monster is supposed to deal 20 points of damage, on average, with an at-will attack. Also, it’s a complete mystery how well the NPCs will be able to keep up with the PCs if they don’t have healing surges or similar long-term resources.