It is true, nothing says you can't have both. Greater Aspect says it acts like Aspect, not that it modifies Aspect. Even the Unchained rewrite didn't modify this.
Mind that you still must qualify for the evolutions, so double dipping is likely not possible at all; you still have the evolutions you gained from Aspect.
They stack as much as evasion and improved evastion do... i.e. none at all.
Actually, Evasion and Improved Evasion stack. They just do so in a way that is completely redundant and pointless.
Aspect and Greater Aspect don't stack in a redundant way, though I would certainly prefer if Greater Aspect said it modified Aspect instead of functioning like it.