I get you're both just trying to explain what you believe the official rationale is, but it's stupid and unscientific.
Three weeks after the first shot
Pfizer has 90% efficacy.
Two weeks after its single shot,
J&J has just 66% efficacy.
If 66% is enough, then Pfizer vaccine recipients should be able to bypass testing 3 weeks after the first shot.
If 90% is not enough,the Johnson & Johnson recipients should not be able to bypass testing at all.
This is why people don't listen to the health policy experts. For all their claims about following the science and evidence, they just do whatever the fuck they want.