what happens if 2 pkm have the same speed en both use quick attack?
This is whats called a Speed Tie, an if I remember correctly its completely random which Pokemon goes first.
What would happen would be a speed tie, where there is an equal chance of either Pokemon going first.
The Speed stat determines how quickly a Pokémon can act in battle. Pokémon with higher Speed will make a move before ones with lower Speed under normal conditions; in the case that two Pokémon have the same Speed, one of them will randomly go first. Deoxys in its Speed Forme currently has the highest Speed among all Pokémon with 180. Ninjask currently has the highest Speed of all non-legendary Pokémon with 160. The average Speed Stat is 66 for all Pokémon, and the average for all fully evolved Pokémon is 78.
Bulbapedia
It is randomly selected which Pokemon moves first so any Pokemon has the same chance. 50/50
Source: Bulbapedia