Britain's Norris is second overall and 62 points behind triple world champion Verstappen with eight rounds remaining, plus three sprint races, while Australian Piastri is fourth and 106 off the lead.
There are a maximum 232 points still to be won in the drivers' championship.
McLaren are chasing both titles and could go top in the constructors' standings at this weekend's Azerbaijan Grand Prix in Baku, with the team only eight points behind once-dominant Red Bull.
"The overall concept is we are incredibly determined to win, but we want to win in the right way," Stella told the broadcaster.
"We (will) bias our support to Lando but we want to do it without too much compromise on our principles."
"Our principles are that the team interest always comes first. Sportsmanship for us is important in the overall way we go racing. And then we want to be fair to both drivers."
Stella said McLaren wanted to avoid a repeat of the situation in Monza where Norris lined up on pole position with Piastri second but ultimately lost out to Ferrari's Charles Leclerc.
In that race, Piastri overtook Norris at the second chicane with Leclerc seizing second place from the Briton, who had to slow, before winning on strategy. Verstappen finished only sixth.
"The team interests comes first and these are the situations that above all we need to fix because eventually, as a matter of fact, the way we entered the race in Monza left the door open (to) this situation," said Stella.
The former Ferrari engineer said both his drivers had accepted the situation.
"The conversations have been very collaborative," he added. "Even when I said to Oscar: 'Would you be available to give up a victory?' He said: 'It's painful, but if it's the right thing to do now, I will do it'."