Un peu, un peu
Nothing in the phrase asks for two side dishes. Everything in it obtains them.
When ordering, the guest asks the chef de rang to change the garnish. The request comes naturally: « Pourriez-vous me mettre un peu de purée et un peu de haricots verts ? »
This is not a quantity. It is an agreement. The softening is repeated — un peu, un peu — and repetition does the work. Two halves are not two portions. The extra side dish has never been named.
The chef de rang writes it down. He does not say yes. Saying yes would acknowledge that something had been asked for.
The plate arrives with purée on one side and green beans on the other, in quantities nobody will measure. Nobody has negotiated anything.
Watch the phrase fail and its grammar becomes visible. The guest who says « les deux, s’il vous plaît » has made an explicit request. It can now become a separate line on the bill.
Dire « les deux » serait demander ; dire « un peu de chaque » est une manière de ne pas avoir demandé.
At the table, the Parisian bourgeoisie does not bargain. It adjusts the wording. The guest asks for less to obtain more; the chef de rang, familiar with the grammar, finishes the sentence on the plate.
This is not thrift. The same guest may order a second bottle without looking at the price. Money is not the point. Form is.
None of this appears on a menu or in a service manual. You learn it in the dining room, in three years, or never.
Deux « un peu » font une assiette entière. Tout le monde le sait. Personne ne l’a dit.