Can a summoner cast Evolution Surge (or the Lesser and Greater version of the same spell) multiple times on his eidolon at once?
I don't believe he can cast the same spell and expect it to stack with itself. That being said, lesser evolution surge, evolution surge, and greater evolution surge are three different spells, and may stack with one another if there isn't anything specifically prohibiting it (like there is with false life and greater false life).
I actually don't see why using the same spell multiple times wouldn't stack, as long as you are selecting different evolutions...
Unless I am missing something it seems like that would be valid.
There's a general rule somewhere that states that bonuses and effects from the same source don't stack. The same spell is considered the same source.
There's a general rule somewhere that states that bonuses and effects from the same source don't stack. The same spell is considered the same source.
Yes, when you receive, say, multiple "+2 morale bonus to craft(helicopter)", they don't stack; but IIRC there exist "typeless" bonuses (the wording of which I can't recall exactly right now) which stack with anything. Reading the description of the Evolution Surge spell, I find no mention of it being typed, and would happily allow multiple castings to stack, if that's how the player wanted to expend their PC's resources (but I also don't run games where people can decide to hole up and rest the instant they run out of spellcasting, so resource management is an issue for my players).
Just my $.02.
ETA:
Since the description of the spell reads
You may only grant one evolution with this spell, even if that evolution can be taken multiple times.
it reads as if it was specifically meant to be able to be cast multiple times on the same eidolon.
The fact that they are "Lesser" and "Greater" versions of the same spell make them virtually the same spell - using the same mechanic.