When 3 or 4, i simply pick the Investigator i'd like to roleplay as. With just 2 investigators, if i know the scenario will be hard to beat, i might try to tone it down by some, picking one or both from this priority list:

1) Investigators which can sustain/heal themselvses/others:
They can take a whole lot more, averaging out some of the concentrated barrage that seem to fall on smaller parties.
a) Carolyn Fern and Vincent Lee. These are the most versatile, from what i can tell. They can sustain themselves and each other indefinitely when paired, given there's time. It makes for a slower play using both. The heavy action orientation means they might run out of time. And they will be terrible at agility tests when paired.
b) Jenny Barnes. Has the most potent self-sustainability: with both clue regeneration and horror reduction, she can deal with stuff on her own and let the other investigator harvest clues as needed.
c) Preston Fairmount. Will assume the role of main item collector to sustain himself.
d) Bob Jenkins. Bring him to a social event; such as shattered bonds. He'll stay sane.

2) Investigators with speed bonuses:
- when speed might be more important than sustainability.
a) Rita Young. One additional space per move action is huge.
b) Ashcan Pete. Pretty much enables the trade action in small parties.
c) Wendy Adams. Is circumstantially more speedy than the average since she cannot be Stunned (-1 action) or Restrained (No voluntary move). Those circumstances got a little less recurrent with the addition of a new condition (mesmerized) from BtT.

3) Special scenario helpers
- I don't want to spoil what scenarios contain what. But if you've played them before, you'll know.
a) Kate Winthrop. Extremely useful in scenarios characterized by monsters with slow movement. Note that her special item doesn't require an action to use.

Any combination of these (or picking at least one of these) should in theory have a better chance of surviving a 2-investigator game. No guarantees they absolutely will, of course. The priority should be taken with a pinch of salt.

Any thoughts?