I'm not speaking from experience with other games. I'm speaking from experience playing Blades in the Dark and running Scum & Villainy, and from the blindingly-obvious mathematical reality that managing stress becomes significantly more difficult with only two players.
You keep stating that you disagree, but you keep not addressing the actual substance of the points being made. Instead, you wave around the game's guidelines like they're an article of faith.
I did, though. You responded by trying to insist that I prove how something doesn't have to be true instead of supporting your claim it was.
But, sure. The strong form of the argument you're putting forward is that a PC has some amount of stress X. So 2 PCs would have 2X stress if pooled together. This is the "obvious math," I think, that 2X is bigger than X (or that 4X is bigger than 2X). Sure, but the rest of your argument supposes there is some average cost to a mission, let's call that Y. If Y is bigger than the pool of X, you contend this is a problem. You further contend that Y is necessarily bigger than 2X such that the GM has to monitor it closely and adjust ganeplay so that Y is less than 2X. And that this is not necessary for larger numbers of PCs.
I disagree that Y is even an intended thing under the rules, and have supported this by showing that no such thing exists in any form in the rules. You say this is ne failing to support my argument, but the reverse is true.
The weak form of your argument is that GMs should be oaying attention to the value of Y so that it doesn't exceed the pool of X (so not fixed, but constantly massaged) to maintain a good game. And that this gets harder with a smaller pool of X. To which I've actually shown this to be contraindicated by the rules text.
So, the weak form is right out and the string form is on you to show there's some expectation of stress cost Y that's intended or a natural outcome of play. I've run for 2, 3, 4 and 5 and I never once paid attention to how much stress the PCs and had no resultant issues. That's the players' job to manage, not mine, and so long as I'm following the agenda and principles if play, and following the fiction, there's zero issue. We play to find out what happens.
Underlying both your and
@Fenris-77's arguments is tge requirement that the GM curate the experience. This is not necessary, and, in my opinion, not the intended result (given it's not once mentioned). If you like it curated, then by all means go for it, but this is neither a requirement of the game nor a natural outcome of it.