"Because both are Red Bull drivers, both are high-skilled drivers, fast drivers and we want to test them for the rest of the season because there is a high possibility that this will be the driver line-up for 2018," Tost explained.
"Brendon is a very high skilled driver, very committed, passionate for motorsport. If we give him a competitive car he will be there and he will also fight in Formula One for success.
"And I hope that especially next year we will bring together a competitive package that he can also fight for victories and good positions," added Tost.
He was then asked about what the future holds for Kvyat.
"He is not anymore with Red Bull therefore he is free to decide whatever he wants to do," Tost added.
Hartley was 13th fastest in a troubled practice this morning where he suffered from engine trouble. He managed to post a respectable fastest lap however while Gasly barely took part because of engine problems in his car.