On Archives of Nethys the First Worlder Summoner summon replacement ability is defined as:
Summon Nature’s Ally (Sp): Starting at 1st level, a first worlder can cast summon nature’s ally a number of times per day equal to 3 + his Charisma modifier. At levels where a summoner would gain a more powerful summon monster spell as a spell-like ability, he instead gains the equivalent summon nature’s ally spell (at 19th level, he can use summon nature’s ally IX or gate). When a first worlder gains a summon nature’s ally spell as a spell-like ability, he adds it to his class spell list (he must still select it as a spell known if he wants to cast it as an actual spell). This ability otherwise replaces the summon monster ability of a normal summoner.
Other abilities like this (for example in the same archetype see the Fey Summons ability) it says things like the following:
This otherwise works like and replaces the X ability of a normal Y.
I am not that familiar with Pathfinder, so I don't know whether that's standard wording that means something or just an individual case.
Now, my question is, since the Summon Nature's Ally doesn't specify (as in the ability that it replaces) that you can't have the Eidolon ability running at the same time.
Is this why the First Worlder Summoner is so otherwise-crappy? Can you cast Summon Nature's Ally from your spell-like ability while your Eidolon is active?
Edit: Oh, I should have mentioned that I'm specifically interested in this for Pathfinder Society, though I'd like a general answer as well.