well, there's actually some evidence that it might not stack. Ingame, it says "A shiny charm said to increase the chance of finding a Shiny Pokémon in the wild" – Pokemon that are bred are not wild. This is a new description of the shiny charm, and therefore the one in 1024 percent chance may only apply for last gen.Blazevoir wrote:
That was confirmed last page... lol.
Also, on serebii, they say that
As of Pokémon X & Y, the rate increases from the standard 1 in 4096 to 1 in 1365.3. It is not known if it affects breeding,in regards to the shiny charm. Lastly, the base chance to find a shiny in xy has improved(from 1/ 8192 to 1/4096). Therefore, in xy the chances, if they do stack, for a shiny through charm + masuda might actually be better than 1/1024.
source: http://www.serebii.net/itemdex/shinycharm.shtml