How to mix strings in Python

I am trying to write a function to mix strings in python but I am getting stuck at the end. So for this example, I have 2 words, mix and pod. I would like to create a function that returns: pox mid

My code only returns pox mix

Code:

def mix_up(a, b):
    if len(a and b)>1:
        b=str.replace(b,b[2],a[2:3])
        a=str.replace(a,a[2],b[2])
        print b,"",a
    return
mix_up('mix','pod')

Thanks!