How to achive true parallelism with thread in Python?

I'm learning about threading library in Python. I don't understand, how to run two threads in parallel?

Here are my python programs:

Program without threading (fibsimple.py)

def fib(n):
    if n < 2:
        return n
    else: 
        return fib(n-1) + fib(n-2)

fib(35)
fib(35)

print "Done"

Running time:

$ time python fibsimple.py 
Done

real    0m7.935s
user    0m7.922s
sys 0m0.008s

Same program with threading(fibthread.py)

from threading import Thread
def fib(n):
    if n < 2:
        return n
    else: 
        return fib(n-1) + fib(n-2)

t1 = Thread(target = fib, args = (35, ))
t1.start()

t2 = Thread(target = fib, args = (35, ))
t2.start()

t1.join()
t2.join()

print "Done"

Running time:

$ time python fibthread.py 
Done

real    0m12.313s
user    0m10.894s
sys 0m5.043s

I don't understand why thread program is taking more time? It should be almost half, if threads are running in parallel.

But If I implement the same program with multiprocessing library, time will become half.

program with multiprocess(fibmultiprocess.py)

from multiprocessing import Process

def fib(n):
    if n < 2:
        return n
    else: 
        return fib(n-1) + fib(n-2)

p1 = Process(target = fib, args = (35, ))
p1.start()

p2 = Process(target = fib, args = (35, ))
p2.start()

p1.join()
p2.join()

print "Done"

Running time

 $ time python fibmultiporcess.py 
 Done

 real   0m4.303s
 user   0m8.065s
 sys    0m0.007s

Can someone explain, How to run threads in parallel? How multiprocessing and thread-parallelism are different? Any help would be appreciated.