I read on Bifurcated Carrots that tomatillos are self-sterile, and confirmed it elsewhere (with exceptions as noted here–there are self-compatible ones, but I suppose the only easy way to find out is to plant only one and see whether you get fruit/seeds or not).

The problem is that I heard the “grow it like a tomato” advice several times and assumed I could follow it in all particulars, when that’s not true. Caveat gardener.

So I’m extremely glad that I planted my second tomatillo seedling in a spare corner of the herb garden rather than composting it. I probably don’t need to move it, do I? They’re only about forty feet apart. I might find space by the dry beans, though. I’ll have to take a look.