Ok, so a Summoner in one of my games just hit 9th level. His Eidolon is a Biped, with 2 arms and Slam (so one slam attack). It wields a Bastard Sword in one hand (took the feat). So it normally attacks twice with the sword, and then a Slam at -5 (it becomes secondary when you add manufactured weapons into the mix).
At 8th level - (STR: 20, BAB: 6, Focus in Bastard Sword, +1 Bastard Sword)
+13/+5 sword, +6 slam.
Now at 9th it gains Multiattack if it had 3+ natural attacks (which it does not). The special ability though states that if it doesn't meet the requirements, it instead gains another attack with a natural weapon at -5.
An eidolon gains Multiattack as a bonus feat if it has 3 or more natural attacks and does not already have that feat. If it does not have the requisite 3 or more natural attacks (or it is reduced to less than 3 attacks), the eidolon instead gains a second attack with one of its natural weapons, albeit at a –5 penalty. If the eidolon later gains 3 or more natural attacks, it loses this additional attack and instead gains Multiattack.
That would mean that he would gain a second Slam normally, but when you add the manufactured weapon in there is that still the case?
+14/+6 sword (BAB increased by 1), +7/+2 slam? Or would the second slam only come into effect when he wasn't wielding a weapon?
Slam (Ex): "The eidolon must have the limbs (arms) evolution to take this evolution... This evolution can be selected more than once, but the eidolon must possess an equal number of the limbs evolution."
Limbs (Ex): "An eidolon grows an additional pair of limbs."
It's not completely clear, but it seems like since you need a limbs evolution for each slam attack, that the slam requires both arms.
If it only has two arms (one limbs evolution) then it can either make one slam attack (which uses both arms) or attack with a weapon it's holding.
However, if you decide the slam only uses one arm, then it can slam with one arm and use the sword with the other. In which case the alternate Multiattack ability would let him make a second slam with the same slamming-arm.
To avoid the slam issue, think of if it had only a bite. It could make iterative attacks with the sword, then bite. Multiattack would let it bite twice, with the same head. So however Slam works, should work the same.
I really wish Slams were a bit more defined.
Last time I went looking for a definition of it, I think the Eidolon's slam was the only indication that it's usually arms based. Although upon further thought, perhaps that was just to limit used full body slams.
In the Universal Monster Rules it sayeth:
Creatures with natural attacks and attacks made with weapons can use both as part of a full attack action (although often a creature must forgo one natural attack for each weapon clutched in that limb, be it a claw, tentacle, or slam). Such creatures attack with their weapons normally but treat all of their natural attacks as secondary attacks during that attack, regardless of the attack's original type.
Thats really the only "rule" I can find about it, and it seems to imply that Slam requires one limb.
-S