With the stipulations the headmistress had put on them, there was only one course of action. Manipulate others into striking out at their enemy.
But who would be so stupid as to act to the extent that these two wanted at the cost of being marked as someone who would act against these two? Slytherins knew powerplays when they saw them, or at least any of the ones competent enough to be of any use. Ravenclaws were smarter in general, and while a long term relationship may find a naive one to be leaned on any sudden interest would give it away. With the war over, Gryffindors were no longer jumping at the chance to strike out against a Slytherin like they once did but were still just as wary of the Slytherin definition of politicking. And what good would approaching a Hufflepuff for plotting an attack do?
So their non-Slytherin teachers were delighted. The two promising students helping out their classmates with their studies. They assisted in the extra curricular activities of their fellows. Instead of bickering and threatening and those rumors of the two trying to assassinate each other that were obviously just rumors, the two remarkable students were finally beginning to behave like blessings to the school instead of curses upon it. The professors that had graduated from that house understood the acquisition of favors for exactly what it was.
Which one of them would be the first to "I'd hate to impose, but if you wouldn't mind just..."? Because then the machinations. One thing keeping the school at large safe would be that no student would want to risk the wrath of Selene or Delphini enough to do something to them just to repay a favor. But that just created the real problem of trying to anticipate what a self-proclaimed diabolical genius would have someone do in order to orchestrate an attack with plausible deniability for the actor, leave alone the mastermind.
Even Snape appeared more haunted than haunting. Potion ingredients were checked, rechecked, and then confirmed. No student would dare sabotage one of the girls' potions. But kindly acquiring the oh so similar but dangerously wrong ingredient because the hapless foil was still a potions student and not a potions expert and had been asked to repay a favor by extending an olive branch of helpfulness? He hadn't been murdered yesterday.
The Studies For Muggles professor went out of his way to explain why certain traditions had become traditions. Laws were just rules, follow them. The traditions were steeped in cultural mores. Such culturally cognizant thinking should have helped any muggle student approaching either of the two girls realize if they were following a piece of advice that made sense as a politeness and not actually doing something magically dangerous. Or in other words, magical favors were not merely favors.
And intelligence is its own curse. As the two socialized more, they inevitably became aware of the specifics of the other's machinations. Of course they had assumed the other would attempt their own plan of garnering minions to do their inevitable dirty work - they were doing so, after all. But the schedules and the friendship circles and the social patterns formed in their mind just as clearly as the magic that flowed from their cores. Sitting on knowledge best used later took its own discipline.