I was wondering if a Barbarian's Rage with an Eye of Gruumsh's Rage would stack to make Greater Rage. It would make sense to me since you can have two classes with Uncanny Dodge which turns into Improved Uncanny Dodge.
The extraordinary ability rage granted by the prestige class eye of Gruumsh says
There's no mention of greater rage (nor of mighty rage, for that matter) in the description of the eye of Gruumsh's rage. He just rages like a normal barbarian. But he spits a lot, and everyone knows spitting is cool.
Only things that say that they combine for greater effects actually combine for greater effects. For example, a level 2 monk who gains evasion and who later becomes a level 2 rogue gains no benefit from having evasion twice. (That's sort of what alternative class feature are for.)