John Gill Commentary 1 Kings 15:25

"And Nadab the son of Jeroboam began to reign over Israel in the second year of Asa king of Judah; and he reigned over Israel two years." — 1 Kings 15:25 (ASV)

And Nadab the son of Jeroboam began to reign over Israel in
the second year of Asa king of Judah
Before Baasha did:

and reigned over Israel two years ,
not two whole years; for he began in the second of Asa, and in the third of that king's reign Baasha slew him, and reigned in his stead, (1 Kings 15:28) .