BQN, 30 bytes^SBCS

{∾•Fmt¨×⊸/+˝(𝕩∾32+𝕩)=⌜'a'+↕26}

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'a'+↕26 The letters from a to z.
𝕩∾32+𝕩 The input string joined with the input string where each codepoint is increased by 32.
=⌜ Equality table between those two strings.
+˝ Sum the columns.
×⊸/ Keep the values with sign 1 (or: remove the zeros)
∾•Fmt¨ Convert each value to a string and join.