I'm making a character that is wielding a Kusari-gama, which is a double weapon that is a flail on one end and a light blade on the other end. My question is for expertise feats like Flail Expertise do I get the bonus to attack and the prone instead of slide when using the light blade end or only the flail end?
Each end of a double weapon is considered its own weapon, with all relevant stats applying to only that end.
Wielding a double weapon is like wielding a weapon in each hand. In the table, the first line of the double weapon’s entry describes the end of the weapon wielded in your main hand. The indented line describes the weapon’s other end, wielded in your off-hand. The two ends of a double weapon can have different proficiency bonuses, damage, properties, and weapon groups.
(From the Errata for Adventurer's Vault, emphasis added.)
Therefore, only the primary end counts as a Flail and benefits from feats relating to Flails, while the secondary end counts as a Light Blade and only benefits from feats relating to Light Blades. All other stats and properties are likewise restricted to their respective ends, as specified by the weapon description.