Scardino, who joined the Spanish group in 2011, has been senior executive vice president and head of global communications since December 2016.
Her role has now been expanded. In addition to her previous job title, she is now also deputy head of the communications, corporate marketing and research division.
She will be based between London and Madrid and report directly to executive chairman Ana Botín.
Strachan joined Santander last summer as communications director – group international communications, arriving from Tesco Bank, where he was head of communications.
His new role of director of media and financial communications will see him take responsibility for all financial communications and media relations in countries outside Spain.
The international banking group has operations across Europe and the Americas, and nearly 190,000 employees. It is also a leading sponsor of Spain's football league La Liga, which last week took on a Netflix veteran as its new comms chief, with a remit to expand the league's global footprint.
In August, Louise Shield joined Santander UK as director of corporate comms. She was previously comms director at FTSE 100 insurer RSA.