Eidolons can get limbs. Lots and lots of limbs. By level twenty, you can have an eidolon with as many arms if you really wanted to, along with weapon proficiency, and take the Multiweapon Fighting route.
So. How does multiwielding greatswords work? If your eidolon has twenty arms, I should think they could wield ten two-handed weapons. Would they all get 1.5x strength to damage, or would the secondary greatswords still only get .5x? Or perhaps they'd get 1x? Do the penalties for oversized off-hand weapons still apply? I can't seem to find any multi-wielding creatures that use two-handed weapons. Only one-handed.
Also, could you multi-wield bows? I should think, with twenty hands and proficiency, that eidolon could fight with ten longbows. Or perhaps nineteen hand crossbows, leaving one hand free for reloading, with the addition of Rapid Reload to make it a free action? Maybe slings? How would strength work regarding slings/composite bows? And how would composite bows work? Still only half strength to off-hand slings/composite bows?