I've seen a lot of YouTube videos where people get two shinies in one horde. It really sucks cuz you have to kill one of them :/ So what's the chance of this occurring, with and without Shiny Charm?

With the normal odds, it is approximately a 1/671,089 chance to find two shinies in one horde, and with the shiny charm, it is roughly a 1/74,565 chance.

Source: https://m.youtube.com/watch?v=wJms4HxaDeg