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First:
Medium Armor class ability gives medium armor proficiency. Unless, you have the spell dancer archetype. The spire defneder archetype prevents you from gaining the proficiencies, thus modifying medium armor class ability.
Ergo, spire defender and spell dancer modify the same abilities and do not stack.
Im sorry good sir but you are wrong in case of RAW.
„Dance of Avoidance (Su): At 7th level, while wearing light armor or no armor, a spell dancer gains a +2 insight bonus to Armor Class. This ability replaces the medium armor class feature.
Greater Dance of Avoidance (Su): At 13th level, while wearing light armor or no armor, a spell dancer’s insight bon us to Armor Class increases to +4. This ability replaces the heavy armor class feature.„
„Weapon Proficiency: A spire defender is proficient with all light and one-handed simple and martial weapons, as well as one exotic light or one-handed melee weapon that has the disarm or trip special feature. This replaces the magus’s normal weapon proficiencies.„
Lets examen both of the archtype features. First with Dance of avoidance it states that it replaceses the medium armor „classfeature“, while spire defender replacese a weapon and armor proficincy. Two different things. Also the medium armor class feature states no prequisits like for example light armore proficiency. Also you can find what exactly gets replaced at archive of nethys which is:
Spire defender: Weapon/Armor Proficiencies; Spell Recall
Spelldancer: Arcane Pool; 5th-level Bonus Feat; Medium Armor; Heavy Armor
Again no similarities. I hope i argued my point for the RAW part.