"My dear Jailer, I write you these lines in six languages. Show them to people who know the languages. Let them read them. If they find not one mistake I implore you to fire a shot in the garden. That shot will show me that my efforts have not been thrown away. The geniuses of all ages and of all lands speak different languages, but the same flame burns in them all. Oh, if you only knew what unearthly happiness my soul feels now from being able to understand them!" The prisoner's desire was fulfilled. The banker ordered two shots to be fired in the garden.
The explanation lies in the conditional and the order of events. the conditional is that if there was no mistake, then there was to be one shot fired. The message did not specify what was to be done if there was a mistake. Silence might suggest the banker had ignored the prisoner's request, so silence was not a reasonable alternative to one shot if there was a mistake found.
Therefore, we must conclude that there was at least one mistake found in the writing. One shot could not be fired. Silence was not a useful alternative. Therefore the banker "ordered two shots to be fired" to indicate (1) the request had been fulfilled and (2) there was one or more mistakes. Thus with two shots he indicated that there was a mistake found.
The order of events confirms this. First the banker receives the request and writings. Then he fulfills the "prisoner's desire." What desire? The desire that people be gathered in the garden to read the writings and that they look for mistakes. Next the banker opts for two shots to show that (1) they had gathered and the writings were read and that (2) there was a mistake(s).