A dual engine printer comprises two print engines, and identical spread data is forwarded to each of the print engines, although each engine prints only one side of the sheet. Each print engine comprises a signature extractor for extracting a signature representative of the spread data it receives, and the printer is able to compare the two signatures to verify that the spread data is identical. The printer is thus able to detect data corruption or alternatively, if the signature extraction is synchronized with the print engage, the printer can verify that both sides are being printed on the same sheet.