Last season, both drivers endured tough spells on different occassions during the campaign.
Grosjean crashed multiple times at the start of the season, but thankfully for him, he slowly improved.
Magnussen's "aggressive" driving sparked a lot of his rivals to become frustrated with the Dane.
But Haas' fifth-placed finish in 2018 is proof of the great job both drivers are doing for the American outfit.
However, Steiner isn't completely satisfied with the duo, explaining to BT newspaper that they need to sometimes look at the bigger picture.
"It's something going on up in their heads," Steiner said. "None of them are good at not being the fastest man in the team. Romain and Kevin have to keep pushing each other.
"They also just have to learn that it is okay to be the second fastest in the team. It is not a disaster if you are not the team's fastest runner every weekend.
"You can win the 'war', without winning the 'battle'."
Steiner also said that the team's target would be to overtake Renault next season as the "best of the rest".
"The goal for 2019 must be to get fourth place in the Constructors' Championship," he said. "I think it's possible. The numbers I get from our engineers are promising, but I don't know what the others are coming with.
"It looks promising. But it is difficult to compare with last year, because the regulations are so different."