Entry location: After completion of the game (at least twice)
After completing the game on any difficulty, play as the other remaining character and beat the game again. After doing so, at the character select menu, you have the option to play as both Isa and Kachi at the same time. The way this works is that you'll play as Isa and when you press the '-' button Isa or Kachi will switch places, for example if Isa is getting low on health you can switch to Kachi who doesn't share the same health bar. However, in the mission "Coalescence" you only switch the mode of fire, and not the characters.
Submitted by sinfulscepter