I don't think the problem with single founders is about generating ideas. It's about not having someone to pick you up when you're discouraged, multiply your enthusiasm, and turn you back around when you've fallen in love with a bad idea.

Either that, or it's just that misery loves company. ;-)

There's no discussion of cause and effect. It could people being embarrassed, for example, something that would hopefully be less of an influence over two co founders who knew one another well, compared with a random group of strangers.

Incidentally, apparently that's the reason why collaborative efforts between musicians often don't work out too well. In their own group, they're not afraid to say exactly what they think, or be highly critical when called for, but with a relative 'outsider', mutual respect gets in the way of that.

In any case your hypothetical two co founders could simply brainstorm on their own...

Unless the two cofounders are smart, and simply brainstorm separately, then meet together to go over the options. Then they net twice as many and are able to more objectively filter the results. (Seeing the problems in someone else's ideas or code is one of the great benefits in having a cofounder - it helps eliminate a lot of wasted time)