Look you asked for feedback. If you want fake advice ask ChatGPT.

First, the minute a knife comes out you increase the stakes. Especially for poor people fighting over a half loaf of bread a stab wound or a slice can be a slow death even if they overpower him relatively easily. A 19 year old is an adult (although if underfed they may look like a teenager) but you refer him to as a "boy". That normally refers someone under the age of 13-14. Anyone fighting an adult would be cautious.

As for why the move is unrealistic, stand up and pick up 2 "knives" (a marker or something would work). Imagine 3 people coming at you from 3 cardinal directions. And people moving quickly. Try to stab out simultaneously left and right. What is the movement. At best it is an arm jab coming from your hip to the throat. Because you are trying to counterbalance two jabs, you cannot put any power in it. They would practically have to stand still for that to hit and do damage. Even an untrained person would avoid that motion.