Dual Talent Specialization achievement is under General category and worth 10 Achievement Points. To gain this achievement hit level 30 and visit your class trainer to activate it by paying 10 gold.
Having 2 talent trees helps you switch between solo play and group/raid play. For example your warrior can have dps talents for solo play and tanking talents for group/raid play. Or your druid can have feral tree for solo play and healing tree for group/raid play. It is a must have!