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No, the Alwars did not consider Venkateshwara to be both Vishnu and Shiva. The verse that people misinterpret as saying that is verse 63 of Peyalwar's Moondram Thiruvanthathi:
tAzh saDaiyum neeNmuDiyum oNmazhuvum chakkaramum
soozharavum ponnANum tOnrumAl soozhum |
tiraNDaruvi pAyum tir...