[Python] Im having problems running 2 functions at the same time

Im a coding beginner who just got into python turtle. I have recently been working on a project where you control 2 characters. They are trying to shoot each other over a river. But i ran into an error where whenever one side fires a bullet and then the other one does aswell, the first bullet just freezes in place until the first one is finished. This ruins the entire game. Ive seen other posts with this "if name = main" thing but i dont understand it.

I have worked out that this is due to a function being called while another is still running. Ive tried multithreading it but it did not work and im way too much of a beginner to understand this. If someone could give me an explanation on how to fix this i would be very happy. (Yes the code is in german but the names of the variables dont really matter).

import turtle


sieg_1 = 0
sieg_2 = 0

kugel1speed = 4
kugel2speed = 4



screen = turtle.Screen()
screen.title("Paintball River")
screen.bgcolor("darkgreen")
screen.setup(width=800, height=600)

#wasd / links
character1 = turtle.Turtle()
character1.shape("square")
character1.color("mediumblue")
character1.speed(0)
character1.penup()
character1.goto(-200,0)

#pfeiltasten / rechts
character2 = turtle.Turtle()
character2.shape("square")
character2.color("red")
character2.speed(0)
character2.penup()
character2.goto(200, 0)

####################################################################
randkröte = turtle.Turtle()
randkröte.shape("square")
randkröte.color("grey")
randkröte.speed(100)
randkröte.pencolor("grey")
randkröte.pensize(30)
randkröte.penup()
randkröte.goto(386,0)
randkröte.pendown()
randkröte.left(90)
randkröte.forward(285)
randkröte.left(90)
randkröte.forward(772)
randkröte.left(90)
randkröte.forward(571)
randkröte.left(90)
randkröte.forward(790)
randkröte.penup()
randkröte.goto(386,-300)
randkröte.left(90)
randkröte.pendown()
randkröte.forward(400)
randkröte.penup()
randkröte.goto(-400,300)
randkröte.pendown()
randkröte.forward(1)


flusskröte = turtle.Turtle()
flusskröte.shape("square")
flusskröte.color("royalblue")
flusskröte.speed(100)
flusskröte.pencolor("royalblue")
flusskröte.pensize(179)
flusskröte.penup()
flusskröte.goto(0,-400)
flusskröte.pendown()
flusskröte.left(90)
flusskröte.forward(800)


###################################################################################



def schießen1():
x1 = character1.xcor()
y1 = character1.ycor()
kugely1 = kugel1.ycor()
if kugely1 == 330:
kugel1.teleport(x1,y1)
kugel1.goto(365,y1)

def schießen2(x,y):
x2 = character2.xcor()
y2 = character2.ycor()
kugely2 = kugel2.ycor()
if kugely2 == 350:
kugel2.teleport(x2,y2)
kugel2.goto(-365,y2)

kugel1 = turtle.Turtle()
kugel1.speed(kugel1speed)
kugel1.color("darkblue")
kugel1.shape("circle")
kugel1.shapesize(0.5)
kugel1.penup()
kugel1.hideturtle()
kugel1.goto(0,330)
kugel1.showturtle()



kugel2 = turtle.Turtle()
kugel2.speed(kugel2speed)
kugel2.color("darkred")
kugel2.shape("circle")
kugel2.shapesize(0.5)
kugel2.penup()
kugel2.hideturtle()
kugel2.goto(0,350)
kugel2.showturtle()


######################################################################################

speed = 20


def hoch1():
y = character1.ycor()
if y < 260:
character1.sety(y + speed)

def runter1():
y = character1.ycor()
if y > -260:
character1.sety(y - speed)

def links1():
x = character1.xcor()
if x > -350:
character1.setx(x - speed)

def rechts1():
x = character1.xcor()
if x < -100:
character1.setx(x + speed)


def hoch2():
y = character2.ycor()
if y < 260:
character2.sety(y + speed)

def runter2():
y = character2.ycor()
if y > -260:
character2.sety(y - speed)

def links2():
x = character2.xcor()
if x > 100:
character2.setx(x - speed)

def rechts2():
x = character2.xcor()
if x < 350:
character2.setx(x + speed)


screen.listen()

screen.onkeypress(screen.bye, "Escape")

screen.onkeyrelease(hoch2, "Up")
screen.onkeyrelease(runter2, "Down")
screen.onkeyrelease(links2, "Left")
screen.onkeyrelease(rechts2, "Right")


screen.onkeyrelease(hoch1, "w")
screen.onkeyrelease(runter1, "s")
screen.onkeyrelease(links1, "a")
screen.onkeyrelease(rechts1, "d")

screen.onkeyrelease(schießen1,"e")
screen.onscreenclick(schießen2)

######################################################################################

while True:
screen.update()
kugelx1 = kugel1.xcor()
kugelx2 = kugel2.xcor()
if kugelx1 == 365:
kugel1.teleport(0,330)
if kugelx2 == -365:
kugel2.teleport(0,350)

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