Flygon's Sand Slammer vs Poison damage

[ProfessorJacob7]

In this scenario, I have an active Flygon with 10 HP left, and it is poisoned. When it goes to the in between turns phase, does one effect happen before another? Or do they both happen at the same time, thus having the Sand Slammer happen at the same time as the poison damage?

 

[PokePop]

The order doesn't matter.
What you are concerned with is when is the KO taken and the Flygon removed.
That is the last thing done between turns, so Sand Slammer damage is placed.