Prestige Flexibility (3.5e Feat)
From D&D Wiki
Prestige Flexability [General, Class]
Every time you take a level in a prestige class that levels a pre-existing class you may choose 1 class to gain special abilities from.
Prerequisite: Qualify for prestige class which levels at least 1 other pre-existing class. Must be able to stack prestige and base classes. Int or Wis 15+
Benefit: When leveling a prestige class that levels at least 1 other pre-existing class you may choose which one of the classes to gain special features from.
Normal: Only the prestige class special abilities are gained when gaining a level in a prestige class.
Special: Special abilities cannot be taken more than once at any level of a single class. Example, Craig is a level 5 Wizard level 3 Lore Master ECL of 8. At next level since these two classes stack Craig may choose to drop his level 4 prestige special of bonus language for the wizard level 9 special bonus feat. However if Craig takes 4 more levels of wizard (ECL 13) he no longer qualifies for the level 9 wizard bonus feat because he has already taken it. However since the 2 class levels stack Craig is then considered to be the equivalent of a level 13 wizard and would qualify for any special features available on the wizard class progression at that level.