Are Simhadri Appanna & Narasimha same gods or different?
Yes. Both are same.
Appa means "father" in telugu. Word "Appa" is added as Narasimha Swamy is father to all. So, He is also called as "Appanna" .
Chaganti Koteswara Rao, well known scholar in Telugu states, explains the meaning of word Appa and reason for adding it in this video (in telugu).