"Next year, the simulator will be running virtually around the clock," Dr Helmut Marko told Bild newspaper. "(Sebastian) Vettel can't do it alone."
Red Bull confirmed in a media statement on Friday that 25-year-old Swiss Buemi, and Portuguese da Costa, 22, will be the joint test and reserve drivers.
"Next year sees the biggest change to the regulations for some time and the return of multiple in-season tests," said team boss Christian Horner.
"To therefore be able to call on two such capable drivers is of huge benefit to the team."