Compartmentalized Mind is a mental, exotic advantage, described in Basic Set, p. 43. For vehicles built as characters, the two variations Controls and Dedicated Controls are available. Compartmentalized Mind is referenced in GURPS Powers™, p. 44, on its interaction with the Projection enhancement/limitation, p. 67, on its interaction with the Possession advantage, and p. 81, on using the Telecommunication advantage with it.
- On Contentrate being the only extra combat maneuver it allows, and on reducing casting times of spells 
Extra Attack does nothing unless you're taking an Attack, All-Out Attack, or Move and Attack maneuver. If your "main action" that turn is to Concentrate on a mental ability, your Extra Attack does absolutely nothing.
Compartmentalized Mind works regardless of what maneuver you take. So, no matter what you do on your turn, you get an extra mental action. You'll notice that it costs twice as much. 
Psionic Powers page 13 introduced some:
- Limited: 3 versions
- Limited, One Ability (only if more than 1 ability in a power)
- Limited, One Power
- Limited, Psi
Pg 14 continued:
- No Mental Separation
Psi Only -10% No separation -20%