I triggered the fireworks display, then I went to the landing zone of the helicopter and prepared with my sniper rifle.
Dalia enjoyed the fireworks show on the third floor and Viktor watched it in the yard. I took them out in less than 5 seconds when I confirmed that they were watching the fireworks but no challenge accomplished.
What’s wrong?
The challenge also requires you to not be spotted, which means even if you aren’t spotted until the double kill, you will still need to finish the mission by using an exit and not to be spotted for the entire mission.
In short, try it again and finish it completely without getting spotted.
另外，我理解你无法使用 YouTube 观看教学视频的困扰，但还是希望你能使用以下讨论串来询问问题。不过若你还有其他问题的话，也欢迎你留给我私人讯息来询问喔。 
Thanks. I tried again and this time I changed my shooting position to the corner near the stairs to the boat. And I took out Dalia first then Viktor, and then I quickly moved downstairs to avoid any possible spot.
I guess the first time I missed the challenge is because that bodyguards would check around while their VIP is down, with a super long detecting range. Taking out Viktor first would cause the chance that I got spotted.