Upon careful examination, I do not believe that multiple defending weapons should stack with each other. A defending weapon applies an enhancement bonus to AC, not an unnamed bonus. This should stack with magic armour, because that's an enhancement bonus to an armor bonus to AC, but not with itself. Moreover, even if it did grant an unnamed bonus, multiple instances would still have the source "defending weapon" and therefore would not stack with each other.
Also, while an animated shield protects you on command as though you were wielding it, I don't see any way you could be construed as wielding its spikes while it's floating around (there's nothing saying you can bash with it). As such, I would recommend either animation or spikes, rather than both.