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At 4th level, whenever a summoner is within his eidolon’s reach, the summoner receives a +2 shield bonus to his Armor Class and a +2 circumstance bonus on his saving throws. This bonus does not apply if the eidolon is grappled, helpless, paralyzed, stunned, or unconscious.
You may add your base shield bonus (including the bonus from Shield Focus but not including enhancement bonuses) to your mount’s AC. In addition, you may add this bonus when making a Ride check to negate a hit against your mount using the Mounted Combat feat.
As i read it "shield ally" gives base shield bonus and mounted shield uses base shield bonus so it can be used for it. Two feats for a +2-+4 AC for eidolon is... Not so good for use, i think, but i`m just wondering. So can it be?
It`s 3 feats for mounted shield, you need a shield proficiency and shield focus.
I could see it working RAW, would give 2AC from shield ally and 1AC from shield focus, so 3AC to the Eidolon if you ride it.
Might as well cast shield spell for a 5AC base shield ( which won`t stack with shield ally ).
Easier solution, combat reflex / bodyguard / Extra trait adopted + Halfling helpful trait.
RAI: I`m pretty sure you need a physical shield
Shield Ally gives you a shield bonus, but I'm not sure that makes it a "base shield bonus". I think the "base shield bonus" is the bonus you get from your shield itself before magic, feats, and class abilities. Maybe your "base" shield bonus here would be +0 (or non existent) while your total shield bonus would be +2.