Da Costa will be in the car on July 17 and 18 while Sainz, son of the former world rally champion, will drive only on the third day.
The highly-rated Portuguese tested with the team in Abu Dhabi last year and has also served as the team's reserve this season while Swiss regular Sebastien Buemi was preparing to race at Le Mans.
Sainz junior, still only 18, is currently competing in the GP3 series.
Red Bull have a vacancy for next season, with Mark Webber departing, while sister team Toro Rosso will also have a seat going if one of their current drivers graduates to take the Australian's place.
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