Some economists seem to have a second PhD from a U.S. institution, conferred one year after their European PhD. How is this possible?

Note that both professors have their European degree from Bocconi. Double degrees appear to be a Bocconi (and some other universities') special. For the MSc level, it's explicitly mentioned on their web page as a highly selective option; see, e.g., here. But you also find an economics PhD program in which you spend a year abroad participating in a partner university's course work and studies, the AEEP. It is not explicitly mentioned that you'll get two PhDs - and you probably won't on a general basis -, but if you get close to a professor abroad during your year, and start your research with them, it doesn't seem a stretch to think you can negotiate two.

Note that one of the current partner institutions is Northwestern, as in one of the examples you linked, while the other (Berkeley) is not mentioned. This might be due to changes in times, or simply to the professors linked not having been part of this particular program. But as Bocconi is apparently very active in these international cooperations, it seems highly likely that both were part of one of them.

The bottom line is that, in my opinion, it will be very unlikely that you'll find a U.S. institution at which you can get a second PhD in only one year if you haven't been part of such a program, where the second degree is a natural and fairly minor extension of your work predominantly in Europe.