Handle multiple keypresses at once in Turtle

Question

I've got a (hopefully) simple question. I want to script something, that lets two users move their python turtles with keypresses. My script part:

#Forward
onkeypress(lambda:turtle1.forward(1),"Up")
onkeypress(lambda:turtle2.forward(1),"w")
#Backward
onkeypress(lambda:turtle1.forward(-1),"Down")
onkeypress(lambda:turtle2.forward(-1),"s")
listen()

If I do this, one turtle stops moving, when the other one starts. So "listen" does only listen for one keypress at once.

What to do if I want to handle two key inputs at once?