From https://www.sps.ed.ac.uk/sites/default/files/pictures/ekja03/Mining25.pdf page 10:

The bitcoin system is therefore designed to ensure that it takes around ten minutes on average before any miner anywhere manages to discover a nonce, or pair of nonces, that generates a hash with enough zeros. /…/

What is a pair of nonces and how does a pair of nonces solve a block?