SELECT "Unique Patron ID",
MIN(CASE WHEN Store = 'STORE_1' THEN Account# END) AS Store1_Acct,
MIN(CASE WHEN Store = 'STORE_2' THEN Account# END) AS Store2_Acct
FROM tbl
GROUP BY "Unique Patron ID"
You could also use the PIVOT operator for this, but the pattern above is more flexible.