When Orwell said, "When once," it meant the same thing as if he had simply said, "Once," but with added clarification. While it may seem redundant, it really isn't. In the text you provided, the "when" clarifies that Orwell didn't mean that "they" did it once, or one time, and the "once" clarifies that Orwell didn't mean while they did it, or they did it simultaneously, which "when" can mean.
The phrase "when once" isn't in any dictionary that I could find, but this meaning can be sourced because "when once" begins a verse in the King James Version of the Bible:
When once the master of the house is risen up, and hath shut to the
door, and ye begin to stand without, and to knock at the door, saying,
Lord, Lord, open unto us; and he shall answer and say unto you, I know
you not whence ye are...
Having attended four years of seminary and spent two years as a missionary in Portugal, I know that the protestant translation of the Bible in Portuguese that exactly corresponds with the King James Version is the João Ferreira de Almeida version, which reads:
Quando o pai de família se levantar e cerrar a porta..."
The Catholic version of starts:
Quando o dono da casa se levantar e fechar a porta...
The first two words, "Quando o," translate to mean: "When the." In both primary versions of the Portuguese Bible, these same words as the King James Version are translated merely as the word "when."
As a missionary, I spent about eight months teaching Spanish speakers rather than Portuguese speakers. The Protestant version of the Bible in Spanish that exactly corresponds to the King James Version is the Reina-Valera Version starts the same verse as follows:
Después que el padre de familia se haya levantado y cerrado la
The first words in the Spanish version of Luke 13:25 translates to say: "After the..." or "Once the..."
Based on this same phrasing appearing in something as fastidiously translated across languages as the Holy Bible, it becomes clear that the English phrase "when once" doesn't nuance a meaning different than "when" or "once" used by themselves, save for the clarification that using both together creates by eliminating unintended meaning that could be gathered from using either "when" or "once" by itself.
If the phrase "when once" truly added some tertiary meaning that neither of these words by themselves mean, then you would see that reflected in the words of the Bibles of other languages. For example, the Portuguese version might read, "Quando de uma vez..." (When once...), and the Spanish version would read, "Cuando después que..." (When once...). However, that is not what they say.
So, why did George Orwell use such stilted and archaic language? You'd have to find commentary from George Orwell, which I could find none. I'd surmise that he was either trying to be abundantly clear or was trying to sound lofty by employing a rhetoric that made what he was saying sound somewhat scriptural. Regardless of what his reason was, suffice to know that "when once" doesn't denote or even connote a meaning that is distinct from simply saying "once" or "when" alone; rather, it only narrows the meaning to the one that both words share.