By the way, I am not ignoring your "if I cook two identical curries ..." gedankenexperiment, it's just that my brain hurts every time I try to reconcile it with my own hypothesis ...
It has stopped hurting

All is now blindingly clear. No-one is suggesting that if you make $n$ 1-person curries you will need less than $n$ portions of chilli, regardless of whether you are planning to serve them individually or pour them all into one huge
bain marie. The suggestion is, rather, that if you try to cook the $n$ portions in a single pan. all at the same time, then you will
not need $n$ portions of chilli but rather less. The exact amount needed may be of the order of $n^{n/{n+1}}$ {1,00 1.58, 2.28, 3.03, 3.82, 4.64, 5.49, 6.35, 7.22, 8.11, ...} but no-one really knows. So, since you have already told me it is not a
gedankenexperiment but a real one, all you have to do is to cook three curries : two 1-person curries, each with the normal amount of chilli, and one 2-person curry, with twice the amount of chilli. Then pour the two 1-person curries into a large pot, and get as many people as possible to tell you whether your 2-person curry is the same heat as, less hot than, or h0tter than, your two 1-person curries that have been poured together. Oh, and report back !
** Phil.